When the intruder alert goes off, a security guard come to the bridge. This makes sense. What doesn't make sense is that once he makes sure there's no intruder on the bridge, he faces away from the turbolift: the only entrance onto the bridge that an intruder could come through. Sure enough, Van Gelder comes in behind him and knocks him out before he can respond. The guard fails to respond despite the fact that he should be at attention.