I like the idea above regarding game cameras. To take it a step further, you can sometimes catch deals online for wireless home security cameras that use IR (read as "no flash") so they work in the dark but don't tip off the offender. The benefit is that they are smaller than a game camera, they can take video, and they don't have to be stuck to the side of a tree/post where the perpetrator will just see it and take or destroy it. Stick them up in the trees to cover random campsites, have them record back to a hard drive at Tim's house, and see what you catch.
I also like Maillemaker's dye-pak idea!
I'll say this much, they're either very brave or very stupid to be breaking into campers of an organization where EVERY SINGLE MEMBER is trained in the use of firearms.
Competitor # 10797V
R. Preston Chew's Battery, ANV