At the climax, some long shots of the helicopter show the right side's door missing before the fight between Barrett and the pilot knocks it off.
When Barret first enters the vault, he dials top combination lock. The door has two locks requiring a second combination (held by another person) to open. This is common in high security, i.e. 'No-Lone' zones to prevent one person from accessing the area by themselves.
It is stated that the guard dogs at Station 3 are trained not to bark. This makes no sense, as a barking dog would alert its handlers to an intruder, thereby greatly increasing the chance of capturing or turning back any trespassers.
It might have been better for all concerned had the helicopter pilot attacked Barratt after the helicopter landed.