You can put the number in your battery compartment according to the link. So exactly how is someone going to "lift" your number (unless they randomly lift it)?
There's an exception to everything. Not sure about yours, but there's not much play in my battery compartment of my p3. Putting anything in there, sticker or paper is not going to work. I'm sure plenty of people will put them on the outside of their machine, if space allows. I was just throwing out an example. You understand what a hypothetical is?