There are other relevant considerations in Europe. For example, the working time rules could make it reasonable for an employer to at least require disclosure o

There are other relevant considerations in Europe. For example, the working time rules could make it reasonable for an employer to at least require disclosure of any other paid work and potentially a power to object if it would leave them in violation of the law through no fault of their own. There's also an important distinction between work done during the period of someone's employment and work done in the course of someone's employment, if you're using the terminology common in the UK and I think various other English-speaking legal systems. The latter is, IME, commonly accepted as a reasonable standard, and roughly speaking means anything you do on work time or using work resources belongs to work. The former is a much broader claim and potentially includes anything independent of work as well, and is the red flag that should give any prospective employee pause.