Possibly a former employee, but it feels like he isn't supposed to be there, like the employees don't know he is there.
I've gone back and forth about it...with as much control as they seem to have, I can't imagine they have someone wandering around in there, outside of their view...which is why I think they know he's there, they're just looking the other way...just sort of speculating on the circumstances...unless he's found a way to hide from their sensors...