I would ask the cops in your local area what their suggestion is. Short of welding or brazing something on the bike, all the suggestions are easier to fake than the lock and key and bike lights. If they say registration, ask them if the cop would be able to call it in or would it only be useful if the cops impounded the bike. I don't understand that a locked bike would be considered abandoned and someone would call it in so quickly, who would bother? Two cop cars coming by in such a short time, must have nothing better to do. It was likely a strange coincidence.