Maybe they've just reported it to their insurer and passed over the details you've left. Have you reported it to your insurer?
Or maybe the paper blew off, someone took it off, or they simply got into their car without noticing, turned on the wipers and the paper flew off. (The Mrs did that once with a Council parking ticket - didn't notice the plastic wallet, set her wipers going as she drove away and then thought "What was that? Something's just flicked off the screen - looked like a parking ticket!"
She contacted the Council and was able to get it all sorted out).