We try not to have them come to the back shop, but sometimes it's just necessary.
As I'm leading them back, I'll say something aloud like, "guys I have a client coming back". Makes the client realize that it's not the norm, and maybe we have "sensitive" things going on.
We get them out of the back as quick as possible.
Nobody and I mean nobody walks past my little swinging "half-door" that says "employees only" on it. Our back door has a message with stronger language. We definetly don't want customers just using the back door.
When they drop off a vehicle for lettering we usually offer them a ride back to their work. Sometimes they'll say, "no thanks, I'll just sit inside the vehicle and wait".
I say, "no you won't. Can I take you somewhere?" "There's a good coffee shop two blocks north of us", that kind of thing. Geesh!