FWIW, a T&P valve leaks for one of three reasons, two are safety, one is that it is defective (or gummed up, effectively defective). The two safety factors are excessive temperature OR pressure. If I had to guess, I'd guess one of your expansion tanks is shot and your pressure is still spiking after hot water use then during the recovery. If you knock on them, each should be nearly empty and sort of ring rather than thud. Also, the only way to check the pressure on these is if you've shut the water supply off, then opened a hot faucet until the water stops running - then, while the faucet is still open, you can check and adjust the air pressure. That part is easy - a tire pressure gauge and a bicycle tire pump. If you get water out of the air valve, the tank is shot. The pressure should be the same as what your PRV is set to. The shaft and or seals on a T&P valve can get debris caught on or in them, and that can cause them to leak. At that point, you need to replace them.