According to David only he knew of the existence of the spare key for the trigger lock, let alone where it was hidden. The spare key was used by the killer to unlock the weapon. There is no evidence that Robin knew either of the existence of the key or its whereabouts. It is therefore a powerful inference that David unlocked the rifle on the Monday morning.
On two occasions David Bain testified that no one else knew that there were two keys for the trigger lock and that no one else knew where the spare key was kept.
The rifle was kept locked, so this means that only the person who knew where the spare key was located was able to use the rifle.
When police entered David bains bedroom on the morning of the murders the trigger lock with the spare key still in it was located on the floor with some live rounds.