In v3, the renderman's face changes when he sucessfully sneaks up on you. (GrinMan to asciiTerror)
This can also be reversed so that grinman only appears when he sneaks up on you. It's worth bearing in mind that rendermen will not allow themselves to be caught in your field of vision at all unless they plan to kill you in the next 10 minutes.
If a renderman comes up behind you, your player will also become frozen and a creepy laugh sound will play (akin to the monster from super 8)
v3 should be a while yet though. I'm rewriting the entire thing, and that's never easy.