I have been finding water under the kitchen sink every few months. I had a plumber come out to inspect under the sink. He found nothing out of order. About a month ago I open the cabinet door below the sink and it was filled with about an 1/2" of water. I still could not find where the leak was. Now for the past month I check under the sink every day hoping to see a drop of water on a pipe or something. Everything is dry. What could the problem be?