I had one do this at a church, only the service was indoors.
The lady that called argued loudly that it was coming from a box on the property (which was lower than the service).
She finally admitted... That was where the plumber told her it was coming from.
(Not the utility vault, which was 10' higher than the service)

Go figure. I thought all plumbers knew that water flows downhill. [Linked Image] [Linked Image]