Sometimes this secret second job goes beyond mere work hours.
It is not unusual for people working with expensive (but moveable) equipment to be approached by other employees to work for them.
For example, the maintenance crew that cleans and waxes floors may be asked by another employee to do the same at the employee's home (or other business), using the company equipment and supplies.
This covert use of the company equipment is "conversion," I think, but not "theft."
One of my friends asked a worker in a window repair shop to come to her home and replace some of her windows (but he refused.) She was interested in hiring his expertise -- not in having him steal glass.
One of my friends is a nurse, and she was hired from the hospital she works at to give (on a private basis) short-term care to a sick person in his home.
Another friend of mine approached the guardian of an elderly person she was working for as a Home Health Aid and suggested that they hire her separately from the company she worked for, saving them money and giving her some benefit or other (which I forget right now.)
Floor cleaners, window washers, repair experts, painters ... workers like that are the people frequently solicited because of their expertise or for the use of company equipment or supplies.
Most often this kind of moonlighting is paid for in cash, and the IRS has no knowledge of it. It's one of the main contributors, so I understand, to the "underground economy."