Check the water meter
Turn off taps and water-using appliances, wait until toilets have finished filling, and observe the meter. If it continues to move, there may be water flowing somewhere. This test does not locate the leak, and some properties have shared or complex supplies, so treat it as an indication rather than a diagnosis.
Look for changes inside and outside
Inspect cupboards, skirting boards, ceilings, walls and floors for dampness, staining, swelling or mould. Outside, look for unusually wet soil, lush patches, ponding or water near pipe routes. Do not remove building materials or enter unsafe areas to search.
Is a hidden leak urgent?
Arrange prompt help if water is actively escaping, damage is spreading, the leak is near electricity, or the source cannot be isolated. A slow leak can still damage cabinetry, flooring and plaster or encourage mould.
Tell the plumber what changed, when you noticed it, whether the meter moved and whether the property has a separate irrigation or rainwater system.