Side Mission
An optional, secondary activity that interrupts or accompanies someone's main goal, compared with a side quest in a video game.
“I went out for groceries and somehow started a side mission helping someone move a couch.”
Use it for unexpected errands, detours, mini-adventures, or goals that are not the day's primary objective.
Gaming language about optional missions spread into everyday storytelling through memes and short-form video.
side quest
The more common equivalent.
random side mission
An especially unexpected detour.
Is a side mission important?
It can be useful or memorable, but it is not the main objective.
Can it describe work?
Yes, such as a small project unrelated to the main assignment.