mystery leak in ceiling
Last week, I noticed two stains on our living room ceiling beneath our master bathroom: one about 18"x3" ran along the joint of the sheetrock and cracks were beginning to show along the tape. The other was about 4"x1." Plumbers came out and determined the larger stain was under a double vanity and the smaller was closer to the toilet. After searching for a source in the bathroom, they cut into the ceiling and still couldn't find any active leak. They suggested it could be from the roof since we'd had recently had a lot of rain. (NOTE: it has not rained again since the first stain appeared.)
I called a contractor who came out, looked at the stains, went into the attic and tried to find a source using a moisture meter: nothing. He said it couldn't be the roof since the attic and upstairs ceilings were fine. After he left, I noticed another stain had developed in the living room: a perfect concentric circle about 4 inches in diameter. He has since cut a bigger hole in the ceiling and we still see no source of water. None of the wood appears stained or moldy, and all the pipes stay dry when we run water from anything in the master bath. Also, we saw that the circular stain is directly under a nail in a support beam that has no plumbing above. It's as though the nail dripped and the circle bled out from it.
Finally, while he was here yesterday, another stain appeared.....but the pipes and area above the ceiling are still dry.
The house is only 3 years old and extremely well-built. We've never had any problems before. Now we're about to sell it, and this happens! HELP!