Holly does this as well. She does it quite often.

Out of the blue she will just stare into thin air and stare whineing or growling. Then back of and hide as if she has seen something she doesn't like.

I don't think we have mice as we live in a new apartment.
I know that she is wary of shadows so could be that!!

Let us know if she keeps doing it Dogs and animals are supposed to have heightened awareness of ghosts/spirits - but I wouldn't worry too much about it. I'mn sure there is another explaination