Marooned_n_Aggieland said:
When you say the system is off that can mean off several different ways. For this response I will take that to mean the system is charged with water but you have no zones activated.
Check to insure the value at each zone is completely closing. It only takes a little bit to get the sprinkler head to drop out of sight but then trickle just a little bit. It builds up over time and you get damp/wet areas that don't really go away. This is an easy fix with a valve kit and does not take much time or tools.
If all your values are good and you get a wet area where no values/sprinkler heads are and there is not a wet path to one of these then the problem would get much more difficult.
My PRV was set to 90 psi and about a year ago was diagnosed as faulty. The plumber detected as high as 110 psi while he was there and turned the PRV down from 90 to 60 hoping it would be fixed. I tested it several times since then and it has always stayed at 60 psi, but I believe this is where the leak originated from. The leak is intermittent, so it leads me to believe that the PRV is still failing at times, causing water to seep through inconsistently. Or maybe it is a faulty valve that doesn't close consistently.
I do have a master valve on the sprinkler system, so that narrows down the potential leak points. Unfortunately, I don't know the location of the master valve as it was installed before I owned the property.