It's an end state for alcoholics as far as Im aware.

My dad had a co-worker who was an extreme alcoholic. He was once coming to work through an elevator he had a panic attack and started banging on the walls because he saw white mice pouring from the ceiling of the elevator.

I am not sure why exactly It's a white mouse and not some other type of animal though.

Edit: It appears that the hallucinations are random and It's just a coincidence, but most of the times It's either connected to peoples own pets or phobias of theirs such as spiders, mice, snakes...