Continuity mistake: As Gant is pulling the Firefox out of the hangar, it is still very dark, as in pre-dawn. But as the Russian officers are shown shooting and watching the Firefox take off, it is suddenly much lighter outside as if it is after dawn and the sun can even be seen. But it would have taken 30-60 minutes for the light level to change that much.
Continuity mistake: Immediately after Gant steals the Firefox, the main screen in the middle of the cockpit states that he only has three missiles in the third missile bay, while the others are full. Later on, when he fires on the recon plane, all missile bays are now listed as full.
Chosen answer: There was just an explosion in the hangar and in the resulting confusion none of them were sure why it was moving. To get it away from the fire is not unreasonable. Also, the Soviet military is not big on showing initiative. Unless told to disable it, none of the guards would risk severe punishment for damaging a valuable aircraft.
Grumpy Scot