Most leaks here show up on the water bill before anyone sees them, because they happen inside walls and under floors.
A test you can do in a minute
Turn off every tap and anything using water, then look at your meter. If it is still moving, water is going somewhere it should not.
Grey pipe fails at the fittings
If your home has polybutylene, leaks tend to start at the crimped joints rather than in the middle of a pipe run. Those joints are often inside walls or up in the attic, which is why the first sign can be a stain on a ceiling rather than a drip you can trace.
Found without opening walls
Listening equipment and thermal imaging narrow a leak down to a small area first, so anything we open is small and deliberate rather than exploratory.
Why acting quickly matters
A slow leak stops being a plumbing problem fairly quickly and becomes flooring, cabinets and drywall instead. The plumbing repair is almost always the least expensive part of a leak found late.