I tag some of my stuff, like scissors and pencils, with a piece of red
masking tape. A lot of kids just don't realize they are walking out with
things, or they put them in their "marker box" without thinking. There is
always some intentional stealing going on, but thats not the main problem. I
find that its mostly thoughtlessness. Also, I have found that some things
get thrown in the garbage during cleanup. Watching for that might help.