I agree, but I think it would be far more effective to catch fleeting glimpses of them prior to their second encounter in the warehouse.
As for their original introduction, I loved it. The fact that the security guard is just shot from nowhere makes the player very on edge. When you don’t even have a glimpse of your enemy to put a face to it, all sort of possibilities jump into your head.
Another reason for having glimpses of the assassins before their second encounter in the warehouse is that the player will know how dangerous they are and so therefore, if the player were to glimpse them a couple of maps before their encounter, the suspension will drag on to the point of breaking. The player will hopefully creep round every corner and be on edge the whole way through.
Plus, all of us will have played HL before and know what to expect anyway. It could also be a way of letting the player know what is to come. As it stands at the moment, there’s nothing except a dead scientist and some scary music to let the player know that something is amiss, and a dead scientist is the norm by this point.