[Verginius Rufus]

I would guess that covert agents at least would not have decals which are permanently affixed to a window or bumper, but agents might have decals that hang from the rear-view mirror (and can be hidden in the glove compartment at other times) or maybe a credit-card-sized card or perhaps something like a garage-door opener to permit access to the lot.

[airedale]

I'm not even sure they'd have that. My guess is that they'd be cleared in as if they were you or me. Any kind of identification could be dangerous. On the other hand if Plame had a CIA sticker on her car even if it didn't say CIA on it would show that she was no longer a NOC and it would be evidence that the CIA really wasn't protecting her identity that well.