If you and the majority of the top commenters are young Americans then this has made me double stoked for studying abroad in the states next year.
I am at University in the UK and I have lived in this exact type of living situation for three 1/2 years. Honestly shocked it's taken this long for something like this to happen.
Not that I am saying it is okay, it's not. I've never done it. It's disrespectful as hell and it's one of the worst gripes of living in this type of accommodation but it's pretty much a given here that this type of thing happens. If you're lucky occasionally, if you're not regularly. Depends on the mix of people.
Students especially freshmen are skint, bad at money management, lazy AF and prone to becoming horrendously intoxicated on a regular basis and making bad decisions = stolen food. Especially premium yums or staples.
A lot of things could have happened to your dear pops ranging from an innocent mistake to intentional stealing. If the culprit stayed tight lipped thus far I doubt you'll ever know.
I get it, man. Despite the fact that it's taken with a pinch of salt here there was one time that really got to me. I bought three posh chocolate bars as a treat during finals and within a couple hours of getting them back from the store they were gone. It was the middle of the day too. I raised hell especially as I knew there's no way the person had eaten them already so I thought there might be a chance to save them. I never found out who it was or got them back (despite even saying there'd be no consequences if they were put back anonymously). It wasn't a lot of dough but it had me pissed. I'd feel so achingly guilty because I think its so adorb when you can see your roomies have bought themselves little qt food treats. Like you I could narrow it down to one of two people, but not enough to accuse anyone.
You've made it clear to whoever pulled this crap it really bothered you, that you didn't roll over and accept it, and that you would care if it happened again. Chalk it down this one time to shitty shared student living and get yourself a new pack if they don't mysteriously re-appear soon. If it happens again take further action. But if you've made it this far without it happening (assuming academic dates wise you moved in awhile back) and you've kicked up a fuss I doubt it will.