Not going to knock on this door
If anyone leaves anything on your door, don’t touch it. Don’t even get close to it. It just might be the very last thing you end up doing. This guy had no idea what they were looking at and didn’t know what to do. Instead of calling the police, as one might think of doing in such a situation, they asked the internet was this thing was.
Thankfully they didn’t call the police, otherwise they would’ve gotten a stern talking-to. This is actually a mounting bracket for a door security camera. Perhaps the owner had removed the camera to recharge its batteries.