I would ask the office manager. It's his or her job (at least theoretically) to keep track of things like this mug. If they don't know, and the two of you make

I would ask the office manager. It's his or her job (at least theoretically) to keep track of things like this mug. If they don't know, and the two of you make the rounds asking everyone whether they brought in this cool teacup, it seems like one of those funny quirky stories that would make people think more fondly of their workplace.