Whether the loss is confirmed at all. The slowest possible start is arriving to look for a leak nobody has proven exists. A meter test settles it in thirty minutes: turn off every tap and appliance, don't flush anything, note the position of the leak indicator on your meter and the exact time, wait, then read it again. Movement means water is being lost on the pressurised side. And no movement is the most valuable result of all — it means there is no pressurised leak, so nobody should be searching for one.