I have a mystery leak that keeps draining water into my furnace. While there are various copper pipes running above the unit, none seem to be leaking or showing signs that they were leaking recently. The water shows up intermittently, but doesn't seem to be tied to any specific event. Any thoughs on how to track down the culprit aside from 24hr observation until it happens again? My only suspicion so far is that it may be tied to the exhaust duct.