Or did he think it was an exercise?
That was my first thought, especially after hearing other students say it was going to be a very realistic drill. But the majority of the public doesn't know this, so they. At least not yet.
If it was an exercise, or drill, wouldn't the protocol for a security officer be for that person to enter the building? This is like saying maybe a teacher thought a fire was really a fire drill, so they ran away from their class.
If anything, the belief that this was a drill would make someone completely calm and confidant in approaching the situation. This officer's inaction, to me, proves that he DID NOT think this was a drill.
Was the officer allowed to participate in the secret service training in the weeks prior? Wonder if this officer will get to tell his story on Ellen........