I've had this happen on occasion as well.
Usually the leaks are at the gas poppet, but I've had at least one instance where after a short investigation the problem actually turned out to be the relief valve, the little 'plug' inside having worn down a bit. I wound up taking it apart at adding a schmeer of silicone to that little 'plug' inside the valve that makes the seal (aided by the spring in the valve). After curing the silicone caulk as reassembly , there were no problems with the leak.
Lately though, I have been double-checking the gas poppet whenever I disconnect the gas.
Usually, just pushing it in momentarily and letting it reseal itself does the trick. And if there is still any uncertainty, a drop of water on the top of the post where the poppet is tells the tale immediately.