When an operator goes MIA you will get an opportunity to rescue them during another mission. It can be any stage of a mission 1-3. You can choose to quickplay on the area you went MIA or specifically choose the location.
To rescue the operator you will have to pull them from a tree sort of archeon. It will pull back if you do not shoot the flowers that open up from the branches or shoot the orbs that come down the roots. You have 15 minutes to find and rescue the operator. Once pulled out carry the cacooned operator to the extraction point and place into the container.
This is best to do with friends or players who don't have an MIA operator as in quickplay you could be rescuing a randoms operator and not yours.
I'm new to these guides so please give me tips, other than to say put videos up.