A mouse would not have been able to reach it.

Yup, if that is your evidence of squirrels, then that pretty much
convinces me: you have mice. Or rats.

Do some reading about mice. They can get through the most amazingly
small holes (think: nickle sized and smaller), can jump amazingly far
and high, and can easily climb vertical or overhung or upside-down
surfaces. And they make a racket, too. Go get yourself a $0.50 mouse
trap, and forget the electrocution.