Every year I shutoff the taps in the basement that leads out to the outdoor taps.
A couple of weeks ago I noticed that the relief valve on our hot water heater was leaking. We had the water heater rental firm out to the house 5 times to diagnose the leak and replace various parts during each visit. On their last visit they decided to replace the water heater entirely. Anytime we use the hot water, the hot water heater starts up and the relief valve leaks out approximately a cup of water. This is still happening with the new water heater installed.
Through trial and error I figured out that if I don't close the shutoff for one particular outdoor faucet, the water heater does not leak.
If I close the shutoff, the relief valve on the hot water heater leaks.
This may have been occurring every year but I just never noticed it since I rarely go into our furnace room.