Do not spend time looking for the source while water is running. Close the main shut-off valve, usually where the service line enters the house or near the water heater. If the drip continues afterwards, the water may be from a drain line, a roof, or an appliance that already held water rather than a pressurized supply line.
Water tracking along a ceiling reaches light fixtures and junction boxes. Switch off the breaker for that part of the house before standing under it or touching the ceiling.
A ceiling bulging with trapped water will eventually fail all at once. Put a bucket underneath and pierce the low point of the bulge with a screwdriver so it drains in a controlled way. It feels wrong, and it saves the rest of the ceiling.
A bathroom directly above points at a shower pan, a toilet flange seal, or a supply line. A roof or attic above points at roofing or an air conditioning condensate line. A kitchen above points at the sink drain or dishwasher. Knowing this before the plumber arrives shortens the visit considerably.
Water runs along joists and pipe runs before it finds a place to drop through. The wet patch on the ceiling can be several feet from the actual failure, which is exactly why acoustic listening and thermal imaging beat cutting exploratory holes.
Photograph the ceiling, the standing water and any damaged belongings before cleanup starts. Most homeowner policies cover sudden failures but not long-term seepage, so the timeline and the cause matter for the claim.
A real plumber, a flat-rate quote, any hour of the day.