Finding a water stain source on a ceiling

I will try to be concise (if I can!). I will be visiting a customer next week to try to find the source of the water causing stains on her living room ceiling (I did an initial look see). In a nutshell: three separate stains that are fairly light in appearance, stains are long and thin (indicating that the water is travelling - probably along the joices), stains start several feet from the nearest wall and are several feet apart. While there is a bedroom directly above the living room (the ensuite for the master bedroom is the closest upstairs washroom and it is directly above the main floor powder room), there is no indication that there is water leaking (all caulking is in great shape with no cracks or separations, the floor is ceramic with good grout and the bathroom it self is about 6 feet from where the stains on the LR ceiling begins). In the powder room, there is an access hatch in the ceiling (owner didn't even know it was there!). I was able to get part way in to look around. There is good visual access and again, there are no indicators of water leakage or damage. The topside of the ceiling drywall is perfect, no moistrure on water or drain pipes.

So, any suggestions as to what I should look for? I was planing on making some small holes in the ceiling and using an inspection camera to get a better look in the areas of the stains. I wonder if perhaps the previous owner had water leakage problems and then repaired them without painting the ceiling. Current owner said if so, then this was not disclosed.

Thanks for any feedback or suggestions.

Brett