the lights are on but no one is at home

The lights are on, but nobody's home

A humorous expression used to say that someone is stupid or not paying attention, perhaps because they are not responding to what one is saying. Hello, Pete? Earth to Pete. The lights are on, but nobody's home.
See also: but, home, lights

the lights are on but no one is at home

used to refer sardonically to someone of limited mental capacity.
See also: but, home, lights, no, on, one