Leak detector alarm but no water is visible: what to check

The puck on the laundry floor is beeping, and the tile still looks dry. The app says leak anyway. That mismatch is the job: the detector reported a condition at one spot, and your eyes are reporting something else.

What kind of detector sent the alert

Not every leak detector is the same device. Before you wipe a dry floor, look at what actually went off.

Some units are small battery sensors that sit on the floor. They sound when moisture hits metal contacts on the bottom. Others use a cable under a sink or along a water heater pan. A few watch flow or pressure on the main and ping your phone. Some can close a shutoff. Some only beep in the room.

The name on the plastic doesn’t matter. What matters is the kind of signal.

A floor sensor speaks only for the bit of floor it sits on. Water may have run along a baseboard, under sheet vinyl in a Clairemont ranch, or into a water heater pan in a Mission Valley garage. Contacts can get wet for a moment, then dry, while the alarm keeps going.

A cable can trip from a wet stretch you never see. A flow monitor on the main can trip with no indoor puddle. Night irrigation in Rancho Bernardo, a toilet that never quite seats, or a line under a slab can move water that never pools at your feet.

Don’t assume the app is wrong because the floor is dry. Don’t assume there’s a flood because the phone pinged. Read the alert, note the time, and note which sensor fired. If a shutoff closed, leave it and tell the contractor you’re matched with.

Plumbing Pro San Diego doesn’t sell detectors and doesn’t rate them. A sensor is a heads-up, not a diagnosis.

What an alarm can and can’t confirm

A leak detector alarm confirms one narrow thing: the device reached the state its maker programmed as an alert. For a contact sensor, that usually means moisture touched the probes. For a flow device, it means the monitor saw a pattern it treats as a problem. That’s all.

It can’t name the pipe. It can’t tell you whether the water is supply, drain, a water heater drip, or something that already evaporated. It can’t tell you the leak has stopped. A dry floor isn’t a clean bill of health. Water in San Diego slab homes often never shows as a puddle. It can sit in the soil, stain carpet days later, or warm a tile you wouldn’t notice at 6 a.m.

It also can’t confirm a false alarm. This page won’t invent a tidy cause. If you didn’t see the event, you don’t know.

Check the nearest fixture without opening walls. Look under the sink, behind the toilet, at washer hoses, and at the water heater pan. In a two-story North Park or La Mesa house, look at the ceiling below an upstairs bath.

Check the meter with every fixture off. City of San Diego customers are billed in hundred cubic feet, and a leak you can’t see can still move that meter. If it ticks with nothing on, treat the alert as real. If you already have a high water bill and no obvious leak, bring that history. The San Diego County Water Authority posts regional water-use resources if you’re tracking usage.

Past that, you need leak detection from a licensed plumber, not another pass with a paper towel.

Where a plumber may look for the water source

The contractor you’re matched with will start from the sensor, not from a theory. Tell them which unit sounded and where it sits.

In a slab-on-grade house, common from Mission Valley through Kearny Mesa and much of East County, supply lines often run in or under the concrete. A leak there can wet a sensor at a water heater or a hall closet and leave the living room looking fine. The plumber may listen along the slab, check isolated sections of pipe, or look for related signs of a hidden water leak you skipped: warm floors, water sound with nothing on, a meter that won’t sit still.

In a two-story coastal house, water from an upstairs bath can travel along a joist and show up far from the tub. A sensor in a downstairs closet can trip while the ceiling still looks white. The plumber may trace before anyone opens a wall.

Around appliances, they may check washer boxes, ice maker lines, and water heater pans. Around toilets and sinks, they may check supply stops and drain connections. Outside, they may ask about irrigation clocks and hose bibs, especially on inland lots where landscaping runs at night and a flow sensor on the main can’t tell a sprinkler from a broken pipe.

If the issue is a failed section of pipe, that work sits with pipe repair, not with the sensor. The detector only pointed. The plumber still has to find the opening and make the repair.

San Diego housing isn’t one construction type. A 1950s Clairemont slab, a Mission Valley condo, and a Carlsbad two-story don’t hide water the same way. The plumber should walk your actual layout.

California plumbing work at $1,000 or more generally needs a licensed contractor under Business and Professions Code §7048. That $1,000 threshold took effect January 1, 2026. You can confirm a C-36 license, bond information, and complaint history in the CSLB license lookup. The database is down Sundays 8 p.m. through Mondays 6 a.m. for scheduled maintenance.

How a device alert differs from professional leak detection

A home leak detector is a tripwire. Professional leak detection is the search that comes after.

The device sits in one place and waits. It doesn’t map the piping. It doesn’t isolate a bathroom group. It doesn’t tell you whether the wet is on the house side of the meter or the city’s main. It doesn’t camera a line. It doesn’t decide whether a repair needs a permit.

The plumber you’re matched with works the other direction. They take the alert as a clue, then test. Methods vary by house and by contractor. Some listen for water in a line. Some isolate sections and watch pressure. Some use a camera where a drain is in play. This site doesn’t publish a ranked tool list, and it doesn’t promise that any method will pinpoint a leak on the first pass. Ask the person on site what they plan to use and why it fits this building.

That difference matters in San Diego because so much piping is out of sight. Coastal moisture, hard imported water, and slab construction all make “I don’t see a puddle” a weak test. City drinking water averages about 16 grains per gallon, with variation by location and demand. Hardness doesn’t prove a leak. It does help explain why fittings fail in ways a floor sensor never sees.

A detector can still be useful as an early ping. It isn’t a substitute for a licensed inspection, and it isn’t proof you should skip one.

Cost is a separate question. There is no official San Diego price list for leak detection. Labor and access vary by house. Plumbing Pro San Diego doesn’t set fees. The plumber you hire does. Ask for the scope in writing before they start. If the aggregate contract will be $1,000 or more, licensing rules apply. Even smaller jobs can still need permits for plumbing or gas work. Confirm that with the contractor and with the city or county that issues permits for your address.

A detector isn’t a shutoff plan by itself. If your unit closed the main, leave a note for the plumber. If it didn’t, don’t start taking the system apart because an app pinged. Shut the house off only if you know the valve and you need to stop active flow.

What to tell the contractor you’re matched with

The more precise your report, the less time they spend reconstructing the night.

Give them the sensor type if you know it: floor puck, cable, flow monitor, or a unit that also shut the water. Give the room and the nearest fixture. Give the time of the first alert and whether it repeated. Say whether you saw any water, even a film, and whether it was gone when you looked. Say whether anyone ran a washer, filled a tub, or watered at that hour.

Share meter photos if you took them, with all fixtures off. Share a short video of the beeping unit in place. Don’t move it before they arrive unless it’s sitting in standing water you need off the floor. If you already wiped the area, say so.

Mention the house type: slab, crawl, or two-story. Mention if you’re in a Mission Valley condo with shared lines, a Chula Vista tract home, or an older coastal property. Shared plumbing changes who owns the pipe. In the City of San Diego, the property owner is responsible for the sewer lateral to the main, which may sit past the property line in the street, on an easement, or in a canyon. That’s a sewer fact, not a supply-leak fact, but flag it if the sensor sits near a floor drain or you also have sewage odors.

Ask them to verify they hold a current C-36 plumbing license. Every plumber in our network is licensed and insured. You should still look them up. Ask whether the visit is inspection only or whether they expect repair on the same trip. Ask what happens if they can’t find a source from the sensor location.

Don’t demand a cause in the first five minutes. A dry floor plus an alarm is exactly the case that takes tracing.

If you already called about a plumber in Mission Valley or another neighborhood, keep the same address and sensor notes so the match is clean.

How to request a leak detection referral

Plumbing Pro San Diego is a referral service. We match you with an independent, C-36 licensed plumbing contractor. We don’t hold the trade license, we don’t own the trucks, and we don’t perform the work. You contract with the plumber directly.

When you request help, say you have a leak detector alarm and no visible water. Name the city. Name the sensor location. That routing is how you get leak detection instead of a generic drain call.

Stay on the facts. “Puck under the water heater in a Santee garage, alarm at 2:10 a.m., floor dry now, meter not checked” is enough. “I think the slab is leaking” is a guess. Let the plumber decide what to test.

If they find a failed section of pipe, repair is a separate scope from finding it. If they find nothing at the sensor, that’s also an answer. It doesn’t mean you wasted the visit. It means the alert still needed a licensed pair of eyes.

This page is about the alarm on a dry floor. The hidden-leak guide covers stains, sounds, and meter clues after the beep. The high-water-bill piece covers usage that jumped with no puddle.

When you’re ready, call. We’ll connect you with a licensed plumber who can inspect from the detector out.

When to call us

Call when a leak detector has alarmed and you can’t find the water, or when you found a little moisture and can’t tell where it started. Licensed leak detection is the next step, not another night of waiting on a dry-looking floor. Call us at (858) 400-4417 and we’ll connect you with a licensed plumber.