I know you are saying its a "bomb shelter" but I went back and re-read your original post and it said you had 8 hens killed----the bomb Shelter is Not predator proof and it needs to be so you do not loose more tonight. There is a entry some where you are missing. Good Luck finding it and fixing it.It's a bomb shelter and would hear things moving around when pounding on ceiling.
Seen no other evidence of any kind of critter staying.
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