Tooth-necked Fungus Beetle, another new family for bugguide!

It is indeed Laricobius rubidus.

The thing about Laricobius rubidus is that it's a predator, not a fungus eater. It preys mostly on pine bark adelgid, so could it be possible that it was just hibernating or something in the fungus?

http://everest.ento.vt.edu/~salom/ForEnt/Can_Ent_03.pdf
Here's an article about Laricobius beetles.

Thank you, Crystal, for explaining why I'm striking out at finding another one by pawing through mountains of slimy fungi :-) I suspect the one I found had been hibernating.

I notice the article is about a different species, L. nigrinis.

Yup, it is, but they have very similar habits. We often find random rubidus in our nigrinus colony, because rubidus also eats the hemlock wooly adelgid. (They usually come in as stowaways in the food.) : D