Most leaks in a house announce themselves eventually. The question is how much damage they do before that happens. A drip under a sink is easy to spot, but a pinhole in a supply line inside a wall, a weeping joint under a slab, or a slow seep at the base of a shower can run for months without anyone noticing anything more than a slightly musty smell.
The quiet signs
Before calling anyone, it helps to know what you are looking at. The water meter is the simplest test: turn off every tap and appliance, note the reading or watch the small leak indicator on the dial, and come back an hour later. If it has moved, water is going somewhere. Other clues are less precise but still useful: a water heater that seems to run more than it should, a warm patch on a floor over a hot water line, paint that bubbles or peels in one spot, baseboards that swell, or a room that smells damp even in dry weather.
In Seattle the damp-smell clue can be misleading, because a lot of older houses have basements and crawlspaces that are a little wet for half the year regardless of the plumbing. That is where careful detection earns its keep.
Groundwater or a pipe?
Telling seepage from a plumbing leak is one of the most common reasons people end up calling for help. Water that shows up after heavy rain, along the base of a foundation wall, and dries out in summer is usually coming from outside. Water that appears in the same spot whatever the weather, especially near where a supply or drain line runs, points to the plumbing. Houses on slopes or near ravines and creeks often deal with both, and it is easy to blame one for the other. A good example is the write-up on plumbing in Ravenna, where many basements sit close to the ravine and groundwater is part of everyday life.
How the tools work
Professional leak detection is mostly about narrowing down where to look before anyone opens a wall. A moisture meter maps where materials are wetter than they should be. A thermal camera shows temperature differences, which makes hot water leaks and wet patches stand out against dry framing. Acoustic equipment listens for the hiss of water escaping under pressure, which is particularly useful for lines under concrete or buried in the yard. For drains, a camera run through the pipe shows cracks, separated joints and root intrusion directly.
No single tool gives a perfect answer. A technician will usually combine two or three, along with pressure tests on isolated sections of pipe, until the likely spot is small enough that opening it up is a reasonable bet rather than a guess. There is a fuller look at that process in this piece on catching hidden problems before they spread.
Where leaks tend to hide in older houses
Galvanized supply pipe, still common in houses from the first half of the last century, corrodes from the inside and tends to fail at threaded joints. Copper that was added later can develop pinholes, especially on hot water lines. Old cast iron drains crack along their length, and clay side sewers separate at the joints. Shower pans and tub drains in bathrooms that have been remodelled over older work are another frequent source, because the new tile can hide a failed liner underneath.
Outside, the service line from the meter to the house is easy to overlook. A soggy patch of lawn that stays green in August is worth a closer look.
After it is found
Once the source is pinned down, the repair itself is often modest: a section of pipe, a new fitting, a reset drain. The bigger job can be drying out and repairing whatever got wet. It pays to deal with that properly rather than closing the wall up quickly, since framing that stays damp invites mould and rot. And if the leak turned out to be one failed joint in a run of old galvanized pipe, it is worth asking whether the rest of that run is likely to follow. For more on when a repair is enough and when a larger replacement makes sense, see the post on dealing with leaks in homes and businesses.