My office has a kitchen where the company puts food that everyone can take. People also order lunch, which is delivered to the kitchen.
One of the companies that people order lunch from delivers everything in one big bag without individual labels, and people take what they ordered. When I was new to the company I thought that this bag was from the company and often took food from it.
This happened a long time ago. I don’t know which company it was (they don’t do that anymore or stopped delivering) or who was effected. There has also been some turnover since then and some of the people are likely not working here any more.
What should I do, and can I do it anonymously?
You should donate the value of what you benefitted form to a public cause, that the people affected may benefit from.
See Shulchan Aruch C:M 366:2, Igros Moshe C:M 88, pg. 154