A burst pipe announces itself. A slow leak inside a wall does not, and it can run for a very long time before producing anything visible.
The water meter test
The most useful diagnostic available to a homeowner, and it costs nothing.
Turn off every water-using fixture and appliance in the house. Note the meter reading. Wait an hour or two without using any water. Read it again.
Movement means water is going somewhere. It does not tell you where, but it converts a suspicion into a fact, which is the point.
Your bill
Compare against the same period in previous years rather than the previous month, since seasonal use varies. A meaningful year-on-year increase with no change in household behaviour points at a leak.
This is frequently how slow leaks are discovered, and it is worth actually looking at rather than paying automatically.
Smell
Musty odours matter more in Washington than the general advice suggests. Our humidity means materials dry slowly, so moisture that would evaporate in a drier climate persists and supports growth.
A musty smell in a specific area — a cupboard, a corner, near a wall — is worth taking seriously. So is a general mustiness in the house, which in Western Washington frequently points to the crawl space rather than to a plumbing leak.
Visual signs
Staining on walls or ceilings, particularly if it grows or recurs after painting. Bubbling or peeling paint. Wallpaper lifting at seams. Bulging drywall.
Flooring that has cupped, buckled or become soft underfoot. Grout or caulk that has cracked and stayed cracked. Tiles that sound hollow or move slightly.
Discolouration on a ceiling directly below a bathroom is the classic, and it usually means the leak has been running for some time.
Warm spots and running sounds
A warm patch on a floor can indicate a hot water line leaking beneath it. Hearing water running when nothing is in use is worth investigating — it is easiest to notice at night when the house is quiet.
Outside
An area of grass that stays greener or grows faster than its surroundings can indicate an underground supply leak. Persistent damp on a foundation wall, or efflorescence — white powdery deposits on masonry — indicates water moving through it.
Where they usually are
Under sinks and behind vanities. At the water heater, particularly around fittings. Behind toilets at the supply connection. At washing machine hoses, which fail suddenly and are worth replacing periodically. Behind and beneath dishwashers. Under baths and showers, especially at the drain connection.
In older Washington homes, aging galvanised supply lines can weep at joints inside walls for a long time before anything appears.
Why acting quickly matters more here
West of the Cascades, ambient humidity means wet materials dry slowly. Moisture that would dissipate in a dry climate sits and supports mold growth.
There is also an insurance dimension. Sudden events such as a burst pipe are commonly covered; gradual damage from a long-running leak often is not. A leak found early is both a smaller repair and a better claim position.
What to do
If the meter test confirms a leak but you cannot find it, that is a legitimate reason to call a plumber. Leak detection is a real service, and paying for it is considerably cheaper than opening walls speculatively or leaving it running.