Actually, maybe not so silly. Depending on how quickly floodwaters rise or storm surge comes in, and how fast the water is moving, a car could easily float long enough to be carried some distance away. It could crash into other cars or other people's houses along the way.

Not sure about the rug. Perhaps it's to protect the paint on the roof from rope damage or if small things fall on it. Then again, maybe the owner fears that it'll be underwater when he/she comes back and the rug is something less slippery to stand on in the floodwater? <shrug>