To distinguish these optional missions as one specifically built for this purpose, I coined them "diverts" (which is also the name of this project). The following series of pictures illustrates the life cycle of diverts, although it uses common terminology so that it is easy to appreciate the idea by viewing these images alone.
The example of a divert used in this demonstration is based on MOBA gameplay elements and begins with a large area of effect attack that leaves most units dead except for a slowly retreating and heavily damaged "prize" unit. The destruction of this prize unit should be intrinsically rewarding or provide the player with lots of money or experience, so that they really want to hunt it down. If a similar experience occurs throughout the game with every large area of effect explosion, then diverts could be considered a feature of the strategic combat system rather then an aspect of mission design used in a few instances.