True, but then again, he's only staying at the boundary of the forcefield. It's entirely possible - from his perspective - that the drone malfunctioned, or sent a false positive. Machines aren't infallible, after all. And what if it isn't a response to an alarm being triggered? What if he's just on his usual sentry duty? No need to fuck him up until after we know for certain whether he's gonna find our heroes or not :P