This is a classic example of why you always need an expansion tank when you have a closed system (either with a check-valve or a PRV which acts like one, at least until the internal pressure exceeds the external, IF it has an internal bypass). Add one, and the problem will go away. The tanks can last anywhere from 5-years, to 10 or more, depending on the local water and the luck of the draw. You'll know when it dies...the T&P will start leaking again. Water is not compressible, and when it expands, it has to go somewhere. Often, that may be a leaky valve, expansion of hoses, or if all of those are in good shape, the safety valve (T&P).