**KTMKyd posting**
That's pretty scary, this subject just came up on an Urban Exploration list I'm subscribed to (
http://www.urbanadventure.org for UE stuff). It's pretty fun to do, one the most famous places for caches are the quarries and catacombs under the streets of Paris, if you ever get a discreet trip to the quarries or catacombs (not the tourist part), be sure in look in cracks in the ceilings :D
What kind of stuff does your friend leave around?? I know in the UE community, explorers leave tracts in drains and quarries and such about the UE sport and websites about UE and stuff. It be fun to leave pics of people riding and pro-offroad-vehicle literature tracts on hiking trails and things like that.
-KTMKyd