I am having an issue under my bathroom sink... when the water is running, and when I am using the faucet, there is NO leak under the sink. When I return home after work, or wake up in the morning (after about 6 hours of the faucet being off), there is water under my sink... at least 2-3 cups. I can not figure out where this is coming from? I have had two plumbers come to the house, and neither could solve this mystery! I have a video of this problem and can send it if needed!

The easiest way I know of is feel for dampness on all water supplies and drains. It could also be from under a faucet or it could be such a slow drip around the highest part of the trap that you don't see it in action.