When Mrs. Vikor hands David a gun and he opens it to see if it is loaded, one can see that three of the bullets have been used, as they have the firing pin mark on them.
At around 25 minutes, when David is fighting the alien guard in the stairway to the regeneration booth room, as he tosses the guard's body on the stairs leading down, one can see the mattress the guard falls onto.
Mr. Vincent did not shut off the gas from the fireplace - the first thing that should be done if the gas has been left on.