Roll Over

So last week he rolled over onto his belly from his back, twice. And it so happened that I didn't see either of them. The first time I had him on this back in the front room and I went to his bedroom to get some clothes to change him and when I came back he was just mellow as can be laying on his belly. (He isn't mellow when he is on his belly) The second time I had just given him a bath, and before I went to clean up after it, I told him not to roll over. What does he do! He rolls over. Sad to say though he hasn't done it since.