The leak will be hard to find. Above the dining room are two bathrooms side by side, each with a sink and a toilet, one with a shower and one with a bathtub. Those two things are back to back. No evidence of any of the leaking, no wetness underneath the sinks, etc. which would put the leak inside the walls.
I don't think it's a drain pipe because I hadn't used much water prior to noticing the drip, and I hadn't sloshed any water out of the bathtub afaicr, certainly not enough to reach the floor below.
How do I find a leak inside the walls? Is there some clever way, like putting a radio transmitter in the water that will beep when it gets out of the pipe?
The alternative seems to be to take out the toilet and rip open the wall behind it and look around. "Exploratory surgery".
I don't want to rip out the dining room ceiling because so far there is no damage to it. The water exits through the electric box and runs down the chandelier chain. That's what used to happen 25 years ago when it dripped then, and there was no damage that showed up later.