Baffled by roof leak!

Had a customer bring a 'C' class in with a water leak, resealed the roof as it was bad, repaired the damage from the leak and it looked like we solved it. customer came back with what looked like it was still leaking in the same spot so we checked every thing again, no lights or windows or vents, etc any where the leak was. Couldn't figure it out so we thought it might be from water pouring off the roof of his garage ( flooding the RV roof ) as he parked right beside the garage and under the over hang. He took the motor home home and parked it where he usually does; we went to his home the next day to see, we were inside the RV when we heard a very strange noise from the side of the RV, went outside to see the neighbors sprinkler spray very hard against the side of the RV; the water was going up and being forced into the joints in the metal skin causing the leak. We would have never guessed this would happen unless we had seen it ourselves; so some leaks may be caused by some external from the RV itself.