Originally posted by Bienes Raices:
Originally posted by Dylan Long:
If it's not behind the drywall it could have come from sink water running down the back of the vanity due to improper sealing over the years.
Dylan Long If this were the case wouldn't I see water running down down the wall behind the vanity when I turn the sink on? I don't see any dripping or leaks when I do that.
I was thinking more of the case in which there was water pooled due to an overflow that caused mold to take hold. Yes, generally you'd see an external ongoing leak.
The most likely culprit is a small leak inside the wall, in which case you'll need to open it up a bit and address the issue. To fix it right, you'll want to keep going until you don't find mold anymore.
I disagree with Christian's method for fixing it, but that's mainly because I'd be comfortable "doing it right" without a plumber. I am especially sensitive to mold situations as my GF of 5 years has bad asthma, and mold can be a huge contributor to breathing issues. I wouldn't want to put someone in a home that I wasn't comfortable living in (with her) myself.
If it were me, I'd pull the vanity, cut the drywall out in a 16"x24" patch (between studs) along where the plumbing goes, and explore.
Replacing a panel of drywall that size takes just about as much time as doing one 1/3 that size, so why not get a better view? I'd also replace W/ moisture/mold resistant drywall, rather than just use flashing and glue. But again, this is my preference because I can do it myself, which saves on the cost and time of using a contractor.