"They've been there ages" is not an excuse to eat other people's things!
I would be angry, too.
In our house, we have a treat drawer (naughty treats, crisps, chocolate, sweets etc) which we are all welcome to take from - DSS is allowed so long as he doesn't take the piss. We generally have a healthy diet, but have the snacks there for occasional treats or quick nibbles.
We also all have our own treats, so like DSS might buy something from the shop, or I might, or my partner might. It's etiquette in our house that if it's not in the treat drawer, it's NOT fair game (and nobody is ever going to starve in this house, trust me). I will pay for everyone to have three square meals a day, and provide a reasonable amount of snacks and drinks, but after that, I've always said DSS can buy his own extras if he wants them.
I keep a few Oreo chocolate bars in my office drawer and have a 4 pack, and I treat myself to one every now and again as it's my absolute favourite. Went to get one over the weekend and there were none left. Asked DSS. He said "Oh you weren't eating them so I thought you didn't want them." There is NO time limit on how fast I have to eat MY snacks. He would have something to say if I went into his room and ate sweets he'd bought with his pocket money. I don't even know how he knew I hadn't been eating them, or how long they'd been there, seeing as they were in my drawer, in my office.
Totally unfair, totally reasonable to be pissed off.