Bar Lurker

One who is a regular at a bar or pub, but only sits at the back in the dark.

A bar lurker is a generally mysterious of shady person.

"hey bob, do you see that guy in the back there"

"who? oh that dude in the dark?"

"yeah. i've been coming here for a year and he's never said a word and he's only ever alone."

"That's a regular Bar Lurker right there."