I usually don't take plates, (unless it's a very interesting combination) especially when they are located next to a busy highway like this one. Besides, if you lose a plate in Serbia, you will have to pay a fine for losing it and completely renew your registration and all that is a very expensive process, so I hope the owner found his plate.
You'd better not. The function of a such posture of a license plate like this is to signify it has been found, so the owner could fetch it and mount it back