When Shaw accidentally sets off the alarm in the vault, the mask lowers into the pedestal, but when Hannah is in the vault (being trapped by the alarm being set off), the mask stays on top of the pedestal.
After Hannah's packed up the Nerd Herd computers in the museum control room, she turns to confront Chuck. The second laptop bag, on her right shoulder, disappears and then never returns.
The "oxygen level" indicator in the room Daniel Shaw is trying to steal the mask from starts at 100% and slowly falls when he is locked in. We normally breathe 78% nitrogen and 21% oxygen. A 100% oxygen filled room would be dangerous and possibly even lethal over an extended period of time.
When Agent Shaw was in a room with depleting oxygen he was breathing fine until oxygen level dropped to 0%. In fact he would have trouble breathing and lose consciousness as soon as oxygen level dropped below 17%. No way he would be still conscious and breathing in a room with 1% oxygen left.
The "mask of Alexander" featured in this episode is allegedly the burial mask of Alexander the Great (Chuck is seen commenting on the vastness of his empire). However, the tomb of Alexander the Great has not been discovered yet (it's the Holy Grail of archaeology). What's more, the mask is clearly Mycenaean-styled (it's very similar to the famous "mask of Agamemnon"), a period in Greek history that predated Alexander the Great by about 13 centuries.
At 29:23, Chuck says there is an antidote that may be taken up to "one hour" after exposure. The subtitles read "30 minutes."