Re-watching this scene (one of the best sequences in Star Trek!) I noticed the crew of the Spacedock's control center is mysteriously absent as the Enterprise is heading towards the space doors.
Was there no night shift at Spacedock? It seems unlikely. Or did Kirk manage to lure them away, or lock them in a closet, while he stole the Enterprise? If the crew were present they likely could have prevented the space doors being opened.
The screenplay doesn't mention the control room, but it does mention that the cafeteria is empty because it's night-time.
Since there are no ships on approach, the barn doors are locked and the only two vessels inside the dock are a mothballed ship awaiting decommissioning and a vessel awaiting its shakedown cruise in the morning, there's evidently very little need to have any crew in the docking area.
112 - INT. SPACEDOCK - THE CAFETERIA - (FX SHOT WITH ILM ELEMENTS)
It's darkened now, and empty. A lone cleaning man works among the stacked tables as behind him, in the great window, Enterprise begins backing away from Excelsior like a mouse tip-toeing away from a sleeping cat. The cleaning man looks up in surprise. What the hell's going on?
