I have 3 copper overflow drains on the outside of my house. One is for the hot water heater overflow, one is for the radiant heat system overflow (in-floor heat) and the last one is... I have not idea - and it's leaking. We have an outside swamp cooler and a water softener and no other appliances that I can think would need a drain. In the attached photo you can see the three drains, and the middle one is the one leaking. It stops leaking when we go on vacation and starts again when we are home. There seems to be a correlation with general in-house water use (showers, dishes, washing machine). Total amount is maybe 1/2 gallon a day. Any idea what this drain is plumbed in to?