This is no ordinary tree hole. What distinguishes this particular tree hole from some other tree holes is that this one frequently contains a slug or two.
I've been monitoring this tree hole for about a year now. There is usually water in it. I think the presence of water is what attracts the slugs to it. The humidity inside the hole must be higher and more constant than it is outside the hole. The hole probably also provides some protection against the slugs' predators, if they have any.
The slugs are philomycid slugs (family Philomycidae) that are native to the eastern U.S. I still haven't figured out the exact species we have in the woods around here.