Invisible Ink - UV marker pen
The ideal pen for hiding a message or some co-ordinates. Completely invisible writing - until you (or a victim) hits it with a UV light (which we just happen to have for you) and it glows bright blue. We have experimented with these on wood and paper, and they work really well.
Our supplier tells us that this is a "permanent" marker, we have no reason to disbelieve that, but have not been able to experiment with the pen in all exposure and time situations.