How do you know he knew? It was an educated guess. If Trump didn't show, and he was not detected, he would merely walk out to his car after dark and be on his merry way. What else did he have to do?
As I said before, I used to be a role-player as opposition forces for training our troops in urban combat. As the enemy, I knew the troops were coming, but we had no idea how and from where. I knew their most likely routes because I knew my training grounds like the back of my hand. I could set up ambushes along the most likely routes, and that was usually pretty good. The troops threw us a curve ball every now and then, but I usually knew what to expect. Our job was to anticipate their moves and provide realistic simulations of combat conditions, which included being a sneaky bastard and an asshole opponent at every opportunity. Routh had great tactics and horrible execution and that is why he got caught.