How to find source of dripping drain pipe?
Check all the tanks that hold water - they will have overflows in case of overpressure or in case an old fill-valve starts to fail to close completely.

If the plumbing system has header tanks for cold and/or hot-water, these have overflows for when the fill-valve gets old and fails to close completely. Also lavatory cisterns have overflows for the same reason. 

Water heaters that work with a pressurised supply also have pressure relief valves to vent excess pressure.

All of these can either vent to the outside or to an internal drain.

So I would ask someone to watch the outside pipe while I go around and cause water to come out of each overflow/vent one by one. In lavatory cisterns you can probably hold a float-valve underwater to cause the water to rise above the entrance of the overflow pipe.

It's quite likely to be a lavatory cistern. A slow "leak" on the fill valve won't cause a problem if the toilet is in regular use. It's only when it hasn't been flushed for a long time that the "leak" can cause the level in the tank to eventually reach the overflow level.