I already checked his computer and the side menu absolutely pops up; pressing the Windows key and then P twice will switch to external display only. But he isn't in today, so I couldn't talk to him...

Thanks! We've never used this computer specifically with a projector, but it's entirely possible the user (my boss) is hitting Win+P+P by accident. Sorry, I should have realized that possibility! ...

The computer:
An all-in-one desktop PC with an external graphics adapter to add a second monitor, running Windows 10 (free upgrade from 7).

The problem:
Lately the primary monitor has been...