I wonder why it's out of stock

You dun goof'd, son.

The paper will inevitably show traces of the pen tip that can be revealed using advanced imagery.

That's a gel pen, like the pilot ones which don't make an indentation unless you are writing against a hard surface like a desk instead of a book

Well, not really. The whole point is that the ink appears at first, so the person thinks they are writing everything fine. They might not realize when it disappears 48 hours later.