I used coffee thinking it was the public’s and then notice it said a name on it, I’m. It sure how many cups of coffee I took from it (1or 2), do I have to return the coffee? Could I just put some back into the container? Any issue of ribbis?
You should return the coffee, because it is possible that the person does mind. You should approximate how much you took and add a little bit to that, to make sure that you indeed returned what you took. Ribbis will not be an issue here because you are returning extra, not because you borrowed from him, but because you want to make sure that you returned enough.
Bris Pinchos 1-15.