How can I find leak source when plumber finds no discernible leaks?

Is the leak all the time, or just after rain. If it is after rain, go in the attic while it is raining and see if you see water running down the walls then trace it back to where it comes from, which it could be anywhere, just going down to a low point.

Have you called a restoration company or someone that has an infrared laser moisture meter type device?

These are so frustrating. Water follows surfaces and can run along a long distance before entering the living spaces. The only thing you can do is to start eliminating suspicious spots. Don't look only at areas directly adjacent, but follow any pipes or wires back to another entry point or water source and look for moisture there. If it's a very small leak, you may need to put something in there that will show moisture. Also, look for areas where your insulation is missing. It could be condensation where warm and cold air meet. That would mean no leak at all except for air leakage.

I had one of those in my house that dogged me for a couple years, turns out it was coming in from the roof plate and soffit joint, that we caulked and it stopped it. Had one in the living room next to fireplace, same thing, it was caulking the soffit that did the trick. I have also seen leaks from water seeping in under the window sills that can travel down walls.... again remove old caulk and recaulk. Its now part of my annual routine to check all caulk on windows, soffits, door frames etc. If thats not it, I would rent a moisture meter, and go over all the walls, around windows. By the way, it was an old handyman who found my leak.... dogged persistance paid off.

Have you checked your gutters? Maybe they're clogged or need to be replaced? As others have said, water can travel all different ways. Don't just check a small area, the whole thing needs to be checked. How about the eaves and the peaks in the roof. Are there air vents? It could be getting in there. Or you might need to replace your roof. Good luck, I hope we've helped.

Leak happens once a day, usually late afternoon and up to midnight. I mop and see no more water and interestingly there is no water seepage when I awake in the morning; then again any time between 4p and midnight (sometimes twice). However, nothing to do with rain. We have pulled out dish washer and stove and looked under sink--no water, nothing! Water pooling on kitchen tiles, rising up through grout. Alone here; so going insane! Please help. And thank you for taking the time to offer input.