IF I wanted to remove the residents (I don't).
A couple pieces of carrot by one hole.
A whole egg by another.
A couple pieces of dry cat or dog food by another.
Replace if removed.
Know that mice squirrels chipmunks ect. can be stealing your bait.
You're just trying to condition hole users to nabbing your bait without undue suspicon of a first encounter with it.
Bones fur or feathers near around the entrance could indicate a fox den. But will usually be after pups are old enough to hang around the entrance.
Use same bait that gets taken from your samplings in a live catch trap. Mr. W.D. could build a simple box trap with a drop door. ( Your welcome Mr. W.D.!
).
Skunks in trap get the trap covered with a dark heavy fabric before moving them.
Depending on what's going on , you could have a single resident or multiples.