Rain/irrigation run-off drain pipe opening constant dripping
These sort of leaks can be notoriously hard to track, some of the possible causes can be 


A spring in the area that may have been diverted due to earthworks in the area
Neighbouring property having a water leak that is running into a ground water drain on your property
Having a roof mounted solar hot water system that is leaking
A underground water leak


I noticed you said you have checked the meter for a whole house leak. That is good and I would recommend you do that by turning off all taps toilet cistern taps and washing machine taps etc, then go to your meter write down the reading and leave the test over an hour if possible as very small leaks can be difficult to detect on a whole house meter.