This may be a mute point,
I had the same thing in my gun safe,
It had been moved into the shop untill the new floor in MY room, from the remodel was finished.
My safe has a 3/4" diameter hole about 1/2" from the floor level, unloaded all my gems out of the safe,
moved it into my room, reloaded all the gems back inside and locked it up.
About a week later,
open the door and about fell over, something "Dead SMELL" was over powering, coming from the inside,
unloaded every thing again and checked every thing/where etc.
Couldn't find the source, finally got the bottom floor section pried loose from the floor of the safe,
this floor piece had a spacer about 1" high, that covered the hole on the back side of the safe.
A couple of mice decided this was a new secure cozy place to set up housekeeping and home,
and when I moved the safe back, I slid the safe tightly up against the back wall,
then lag bolted the safe to the wall studs, BLOCKING their front door hole,
they met a untimely death,
along with the smell of that couple of mice dying, that It produced etc.
Did you lift/take out the bottom floor part?
I swear that the smell of Hoppe's#9 gun oil draws them,
this is the second time that those 4 legged hair balls have been in the safe.
Tia,
Don