Personally, I think it is a staff member, but that it is possible that the staff member in question was not seen by other staffs thanks to an inside help, an accomplice. That person could have, by example, a crafted alibi.

Yet even if this is pure speculation, it's worth looking into, all the while avoiding the red herrings.

Redrunelord: *Smiling* What refined manners... (Just keep in mind that thieves can have manners, so it doesn't exclude you of the suspect list. Also, sorry if this seems harsh, I only intended to let the culprit [I AM NOT SAYING IT IS YOU, THE CULPRIT WILL READ THE POST EVENTUALLY] know an act will not work on me. After all, this is interrogations via posts, no? No facial exprressions, correct? For all I know, the guilty one might be laughing as we speak, thinking we are too dumb to find him. He is wrong.)

*Turns around, now facing Elara*

Elara : As for the ink stain... Sharp smell usually comes from sharpies. Yet, a sharpie would not make such a mess. In other words, it is another kind of ink. *Starts pacing*. Now, a probability would be that, depending on the lock mechanism (the last lock's), the culprit COULD have mixed the ink with a chemical of some kind, perhaps a lubricant or an acid: both can have a pretty strong smell. *Halts abruptly*. The problem is... who would have such a product, and was the ink indeed mixed with another substance?

Or was the stain really ink?

We will definitely need an analysis on this stain before we can do progress in that direction.