We're dealing with a shape shifting creature with the minds of very skilled and capable men by the time the blood is destroyed.
There are more then just one way to enter a safe, destroy the contents, and leave without a key. Another possibility is that the key to the safe was useless in the first place as the thing is a sum of its whole. And that whole can, as stated above and in the film's and story can change its shape.
Most safes are not sealed. There is actually a wide gap area between the door and the frame. A very motivated thing, any one of them at the time could have waited for the room to be empty, walk up to the safe and reach inside, reconfigure and get it all in a matter of moments or minutes and no one would be the wiser.
Even if the safe where sealed, that seal would only be ment to keep the environment inside and outside segregated from the other while closed. It keeps the cold in and the heat out. Again, a motivated thing creature would find very little pause to get past that seal when reaching into the Safe with out a key.
The keys being lost would make no difference. It's a red herring that is used as a plot device to further the men's distrust of each other to further muddle the identity of the real culprit.
If a key is all you need to get the safe open, a basic locksmithing knowledge will allow anyone to open the safe with a basic tool. 999
If the safe had a combo, the things hearing and nerve sensitivity could be enhanced so that they could just crack the combo of the safe. If both key and combo is required, it will take just a bit longer but it can be done.
I personally don't even think that the safe was opened at all and it was destroyed while it was still shut. I also believe that the real culprit is likely one of the finger pointer when they all discovered the problem.
By this point there is clearly two or more things in the group. According to the story, and vaguely suggested in the film's. The think has a telepathic communication with each other. If that is true, then they like in the story can telepathically read our unguarded human minds with incredible ease and would be able to suss out the plans of the men with a little digging around a few key minds.
This makes me think that at least a couple of these men might have had a sort of natural latent telepathic resistance or ability that they themselves don't know about. <PERSON> being one of those sort giving him the ability to actually surprise the thing.
The point is that they is more then one way to skin a cat, just because the key is accounted for or not at the time of destruction of the blood, that doesn't mean that t was needed in the first place. It's clear that the humans would not have done it and at the time it happened, the outpost take over was well in progress by the time it happened, and they would. 999