“Work hours” are less clear for salaried workers who may or may not take work home: if it was written to solve a problem for the employer, reviewed with other workers at work, but ultimately not further pursued the status seems murky.

The later derivative that was actively used by and updated for the requirements of another employer during the coarse of work seems to more clearly their property as a derivative (but also murky because it is potentially an illegal derivative of the earlier work, if that was owned by the earlier employer.)

That's not what happened here though. For salaried workers everything you do that is related to your job is owned by your job. That's the default even if your contract doesn't state it. He may not have been directed by his boss to make the code for Box, but he did it with the intent of helping Box's business, as a salaried worker. That makes it Box's property.

But even if you are unconvinced of that, work was clearly done on it on company time at Uber, where it was deployed as part of Uber China's business infrastructure. That work is absolutely owned by Uber (with maybe also some claim by Box). Not owned by OP.

It depends on your employment agreement or contract. Most contracts I have seen say that any IP you develop related to what you're doing at work is the employers.

That story just doesn't seem plausible. Maybe for Box, but it feels like a stretch, and definitely not for Uber.