Did you have anything stored in there? If so what, exactly? How long has it been there? How old is the vanity?

Handprints from urethane glue during assembly. Don't even think about asking brand of glue or idiot's name that got glue on his hands. Prime and paint then forgetaboutit.

@millworkman nothing has been stored in cabinet and sink has not been turned on for 2 years (its in a guest bedroom). The vanity is from the 80s. At one point someone put contact paper on the bottom I have since removed.

@klem1 my guess was some type of adhesive under the veneer as well. I was just hoping it is not mold.

Thank you both for the responses.