Great point 23-down, I think it'd actually be interesting to see the reaction of the "native" Black Mesa military units to the cleanup operation actually. It was something I was thinking of including as an idea in my current mod (albeit at a sister facility to Black Mesa).
The actual pic of the helicopter is directly from the tram ride into the base. You can see it in the tram canyon section, along with Barney waving his arms at it IIRC. In fact it seems all the pics bar the Dam are there are from the tram ride, so clearly they made the texture during the early part of developing the cinematic intro or something.
Also, I think it's clear the military would travel to and from Black Mesa regularly anyway. You'd get military chiefs and so on wanting to see black projects being researched there. In fact, IIRC the HECU was actually a unit assigned to Black Mesa specifically from a nearby base. I would imagine they were initially on hand to provide extra security until their purpose got twisted.
Adds quite a grim undertone to their mission, actually, when you consider some of them may have actually been somewhat familiar with areas of the base and the staff.