I have a sink in my second floor bathroom that is mysteriously filling with water each day. We are not using the sink at all and the faucet is not dripping. The water is clean and clear so it is not used water backing up. The sink is also intermittently blocked or slow draining as well. There appears to be an extra water line or insulated pipe that is attached to the sink trap-it has condensation on it. No other sink in my house has this extra line. What could be causing this and what is the extra line for?

Please bare in mind that I am in no way a plumber!

Actually I don't think it's a mystery at all.

There is a blockage somewhere in front of that sink. Water, hits the blockage (clog) and starts backing up to where it is free to seek its own level, this particular sink.

I suggest you call a plumber and have him snake the drain line for you.

The extra line sounds like a AC condensate drain. If the drain's blocked farther on down the line the AC would continue draining and fill up the bowl. The solution would be to remove the trap and snake the horizontal line in the wall. Good lucik, Tom