Just installed a porcelain under mount bathroom sink. When testing for leaks I can see water seeping through what seems to be a little 1/2 in. circle of what looks like porcelain plug about an inch above the bottom of sink. It comes from the overflow drainage not the sink body itself. I'm not sure how the water is even getting to this area. Is the bottom seal to plumbing leaking then backing up the overflow? No water went through overflow hole. I'm not sure if I need to replace sink. Corian top siliconed to double vanity. Help!