I deleted myself
Your method for judging if someone came isn't full proof if the paper wasn't pressed by the whole door inside and outside the door jam. That way if the door is opened it falls, if you can't open the door, this technique isn't full proof. I would try a small piece of scotch tape door to door jam. I can't tell you what you should do, that's up to you. If you can't get in and don't want to forcefully break in, the choice in abandoning ship is obvious.