I’m suddenly reminded of this joke I heard one time about someone whose lunch kept being stolen. Sandwich, soup, whatever, it was often taken. So the guy put a note on his soup container that said: “I spit in this soup.” Come lunchtime, the soup was still there, but with a new note: “So did I.”
As was mentioned many posts back, the OP should continue with the booby-trapped lunches, but bring his real lunch and keep it at his desk. If he’s accused of trying to ‘poison’ his lunch-thief co-worker, he can pull out his real lunch and claim innocence.
And then ask why the LT is stealing someone else’s lunch. No matter what, if the LT tried to accuse someone of a booby-trapped lunch, he’ll have to admit to stealing it in the first place.
Or, even better, go ahead and claim ownership of the catfood sandwich. Say you bought it at a high-end petstore and that it’s for your cat, Mr. Fluffykins. Act sad that Mr. Fluffykins will now go hungry.
My fear with something like that, is that the alarm would go off, the whole office would walk over to see what it was, and there would be your bad guy (let’s just assume you caught the right person and not just someone moving the lunch or being nosey and looking in it), and you’d dance and laugh and point and shout and make a scene. Then you’d run down to HR and tell them you caught the lunch thief and everyone’s been getting their lunches stolen and everyone saw this happen and now they can finally fire the thief…my fear would be that after all that, HR would roll their eyes and tell you that stealing lunches isn’t a fireable offense, or they’d write him up for it and that would be it (but, hey, catch him doing it 2 more times within a year and then they’ll fire him)
Before I did something like that, I may consider talking to HR first and finding out if there’s any repercussions for stealing other employees food.
I’d hate to make a big scene and then still have to work next to the guy.
Just bring this little guy to work to sniff out the cat-food sandwich thief: http://www.dailymotion.com/video/x751rj_impatient-kitty_animals
Or worse, the OP admits having set a trap and sabotaging his food. The thief then sez “I have no idea of what he’s talking about I just moved this lunch bag to get at the ketchup”, and thiefy is a higher up. OP is fired on the spot- with cause.
I can tell you that cats will consume a LOT of food “packaged for human consumption.” 
Maybe you could make a sandwich with some kind of candy that turns your mouth blue?
Some years ago, we shared a floor with a Govt Dept. The fridge used to get pretty full and often there remained fungus ridden, crappy looking shit. One day I just tossed it all out and cleaned up.
I thought I did a good job and was walking back when I ran into a person from the Govt Dept. I asked where he was going and he said he was about to collect the lunches from the fridge for the visiting Vietnamese delegation.
I just kept walking.
DigitalC, may I offer you this suggestion for any future problems you might encounter:
Scroll to #9
So, how did lunch go this week?
Well so far so good i guess. I still feel like Leonardo DiCaprio in The Departed in that i am not sure exactly who to trust at this point, but my lunch has been left alone so far. As far as i can tell that is, im still slightly paranoid about any retribution attempts.
Brown bags and staples are available from many sources, probably even in your own office. If someone fed me a cat food sandwich, I might go to a little expense to pass off my dog food sandwich back onto them. I might go through the expense of buying some brown bags and staples. I think you should write on the bag too, so you can tell if it’s the original bag or not. Maybe staple the bag in a different pattern for every day of the week? Good luck!
Write on the bottom of the bag, so it’s not obvious. A sticker from home might be better than staples, because it wouldn’t be replaceable in the office.
At work about 12 years ago, I took an ice cream sandwich that didn’t belong to me. The box had been sitting unopened for months so I figured it had been abandoned.
Post #14. 
Thief! Kitten killer!
This is one of the best office fridge notes of all time. OF ALL TIME!
I’m missing something here. 
West got a little crazy at this year’s MTV Video Music Awards.
So- it has been two months. Updates?
Would it still be illegal to poison your lunch if you wrote “WARNING – CONTAINS POISON! DO NOT EAT!!!” all over it? I don’t see how anyone could call that a trap.
My hypothetical solution – make a poison sandwich, and include the appropriate warning labels. If it doesn’t get eaten, you can just start putting those labels on your regular lunch and trust that it, too, will be left alone. If it does get eaten . . . problem solved?
Not having read the (many, many) responses:
I’d make a tuna sandwich, mince the tuna VERY, VERY fine, somehow document that it wasn’t really cat food, maybe video myself making it, then do what you propose. Fuck, if it were me, I wouldn’t even have CONSIDERED that I might get fired or sued, I’d have made just a big-ass Friskies sandwich…
ETA: I like the hot hot hot sauce ideas better.
Joe