I'm guessing your tree is probably fine. The fact you found a beetle under the bark is no surprise since many different types of beetles live under the bark of trees. And, if this bark was loose to begin with any number of beetles might move in. If you had the beetle, I would suggest you take it to your local extension service for identification, but it doesn't sound like you still have it available. Again, if your tree looks healthy and this was only a tiny part of the bark I'd guess you are fine. If you are still nervous about it, call a local arborist to look at the tree and the damage on it. "