.^ That's a daily dilemma I face. I work in a restricted area that doesn't allow recording devices, so I have to cap my camera every time I drive in. A couple of times I've driven 10km home before realising I'm not recording.
I'm going to buy one of these and glue it to the lens cap. Quite fitting I think. With my camera mounted behind the mirror it'd be an easy thing to see when I'm driving and forget the cap.