A gun can be missed, but he should have been cuffed behind his back.
If you are handcuffed behind your back it is not hard to pull your hands over your butt and feet and to your front (assuming that you are young and fairly limber).
It's easy to for some people to slip their cuffed hands to the front. I've seen it done a number of times. The big question is how did he get into the back of a patrol car without being thoroughly searched. Someone will have stand tall on the carpet and try to answer that.