Most leaks here turn 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 that uses water, then look at your meter. If it is still moving, water is going somewhere it should not.
Where they usually are
In the older homes, at corroded galvanised sections and joints that have been repaired before. In the newer ones, at fittings and connectors rather than in the pipe itself. Outdoor spigots are common in both, and they often leak into the wall rather than out where you would notice.
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. In an older home with plaster walls that matters, because making good afterwards is the expensive part.
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.