I like the ink idea.
Also, most people that steal from their "friends" don't act or look like they would steal. Once you catch the thief, my guess is you are going to be quite shocked on who it is. It could be someone very close to you, someone who you might have just given the stuff to if they asked. People do some odd stuff.
What ever your trap is, don't tell anyone at all about it. I would do the ink, because you could tell right then. Make sure it is something that will stain and is hard to wash off. It won't cause any internal pain or could be hazordous as tainted food could be.
Keep us updated.
The people who are my 'friends' are really just aquantinces. I don't hang out with 8/10th's of the team, I just talk to them every-now-and-then, basically friends... but not really best buds - so I'm pretty sure that it's not the 2/10th's of the team I know. I like the ipecac idea, but I'm not so sure now that I found out it's hard to find. I'm thinking of food coloring or lots of ink before I leave out of the lockers
Of course everyone likes you, you give away the greatest stuff!
Hahahah
By everyone 'liking' me, I meant that nobody openly hates me (no obvious suspects, since nobody shows open aggression towards me). I haven't fought anyone or given any reason for anybody to hate me.
Do a survey and find out everybody who has been stolen, and lay an eye on the one(s) who haven't been.
Nobody would do it.
They'd think it was stupid, not worth the time. I'm really like the only person who gets mad about people stealing things. People are mad when they get their crap stolen, but eventually they calm down, and generally don't give two licks of a crap about it anymore. I've had things constantly stolen (who the heck steals opened BEEF JERKY, come on) so it kind of stays with me. Average person has had about... 5 things stolen from them in 3 years, money-wise, around $50 worth at least.