I've seen UML used on whiteboards and done so myself when describing how systems are put together, but that is very transitory and as a communications tool. That's all UML is, after all, is a communications tool.
Like the others before, I don't see UML in design documents or anything related to the game itself on any regular basis.
Rarely someone will document how a system works. They generally don't use UML formally, but with a UML-style collection of boxes and lines to communicate the relationship. Other times the UML communications diagrams showing how data flows over time will show up, but it isn't common.