We have 3 cats, but Atticus is the treat stealer, the other two just wait for the fruits of his labor. In the past, I've had to keep cat treats in the fridge as it's the only place Atticus can't open -- though he knows how, he just doesn't have the leverage needed to break the seal. In my apartment I was keeping them in a kitchen drawer. He learned to open the cabinet under the drawer, pull the stuff down off the shelves, climb on the shelf, push the drawer out from the back, come out, jump on the counter, retrieve treats.
At our last house we kept the treats in the liquor cabinet above the fridge, since the fridge was built in and there was only about 6" hanging out at the top of the fridge. They were safe there... until we started to move. Without so much stuff on the counters and the top of the fridge, he started jumping up there, opening the cabinet, and pulling down the treats. Once my bf's parents were watching them while we were away for the weekend. They bought a canister of some sort of junky cat treats, but I'd left a note saying where their treats were so his parents just left the canister unopened on the table and gave them treats from our collection. When we got home, we found a weird chewed up purple lid and couldn't figure out what it was. His parents didn't mention that they left treats, and we didn't recognize it. A few weeks later I was cleaning and found a torn open plastic canister of treats under a side table in the dining room. They took it and hid it.
We also have to store all dry food in containers with screw-on lids. It's the only kind Att can't open. He has no problem with latches.
Now that we have the 3rd cat who is polydactyl, we joke that if only Atticus could work cooperative with Sir Didymus, he'd be unstoppable -- his brains and Sir Did's opposable thumbs!!