You're not nearly as sneaky as you think you are.
From my library director's FB:
Me to older man at computer: Are you a student here?
Man: Um...erm... I'm *going* to be.
Me: Ah. Well. I've had a complaint that you're viewing pornography on the computer. That's against our policy. I need to ask you to not do that.
Man: Oh. Don't do that?
Me: Yah, don't do that.
Man: OK. [leaves]
I'm almost hoping that he'll turn out to be one of the Usual Suspects from the public library (because it would be hilarious and make for good storytelling).