Just speculating here: No doubt personnel are using a lot more masks at work now. A jump to (say) 100,000 might be expected. That could easily be checked by comparison with other hospitals. My best guess about the remaining 200,000 — they walked out the door, with staff, who want them for themselves and their families at home. I can’t say that I would resist the temptation if I were in their place — but, it shouldn’t be allowed to continue (if that is what’s happening). Normally, such pilfering would be no big deal — but, these aren’t normal times.
I think the hospital admin sold them on the black market for a good profit.