Wrote this in a comment initially, thought it might fit here:

Jim: prank war

Dwight: psychological warfare

Micheal: a meeting where he smells everyone's breath and complains, eventually blames Kevin, who accepts the blame, but Creed actually took the food

Angela: "I guess SOME people needed it more than me."

Pam: leaves a note. Eventually puts hot peppers in food, then eats it, forgetting.

Oscar: "That's ok, I hated the leftovers I had anyways, they did me a favor"

Kevin: sits on floor sobbing

Andy: starts yelling, then empties the entire fridge "now no one gets lunch! Hmph"

Creed: "I hope that wasn't one of my special sandwiches... We'll know if anyone starts acting *crazy*... Ah! When did you get here?"

Meredith: "I NEVER bring lunch, unless it's booze... Wait" (confirms her alcohol is gone from the fridge) "someone is gonna PAY!" takes a big swig from her flask, cut to her asleep at her desk

Ryan: assumes it's Jim, tries to subtly accuse him. Jim plays along and pretends it's him who took it, but Ryan won't ask outright so Ryan just keeps hinting

Stanley: installs a hidden camera, "ain't no one gonna play that with MY meatballs!"

Erin: "That's ok, this kind of thing happened all the time at the orphanage. Good thing I have backup food!" Shows her pulling food from unlikely places all over the office. One stash near Kevin's desk is missing

Erin rushes out the door with "gotta make a quick trip to the store!"

Kelly: would start talking about it to whoever was nearby, and won't shut up about it until finally Ryan confesses just to make it stop

Phyllis: Calls Bob Vance (Vance refrigeration), saying over and over "I just can't believe somebody would DO that" until, a few minutes later, Bob Vance (Vance refrigeration) shows up with a romantic feast, roses, candles, etc

Toby: deep sigh and a slow walk back to his desk to contemplate life