What tricks did he use, aside from hiding in plain sight?
I get that the cop didn't want to be shot in the face, but he WAS there to make sure the ex-Pres was safe, whether he wanted to, or not. I'm not saying he should have forced Crooks to shoot, though- I wouldn't want that for anyone. However, he was boosted up to the roof by another officer, so I think that alerting others was automatic.
But as I posted, people on the ground easily saw him and he could have been taken out within the 20 minutes after first sight.
I suspect the chain of command will be made known and whoever prevented the agents doing anything, failed miserably.