“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.)