Seems like including a knife attack in an operational plan would ensure failure. People are not easy to sneek up on under normal circumstances unless deeply distracted. Presumably a sentry would not be texting a girl friend or other such endeavors. Getting close enough to touch someone without them knowing is nearly impossible.
As kids we played these games many times. When you move they know and turn toward you. The attacker would have to be successful with one decapitating blow from a hiding spot a few feet away. Then you have that chicken thing.