2000 Chevy Silverado gas tank foul play

This morning I find my engin running rough and like it is going to shut off at stops. I then think that maybe there was foul play with my gas tank after finding the gas cap off this morning as well. Initially, I just assumed that someone needed free gas, but with my engin running rough, I thought maybe I should do some damage control and seek a precautionary measure aside from securing access to my gas tank with a locking cap. What would be the sensible thing to do at this juncture?
Bernal