During a Patrol mission right after Crossing Paths (Survive campaign), Mike-26 is marked as an Abandoned Battlefield optional mission. However, the area seems to be marked as "forbidden" - after approaching within approx. 200 meters of the base, Kerry reports that he is lost, and a mortar strike hits his position. I have tried approaching the areas from different directions, and got the same result.
Update: I was able to rush through the mortar barrage on an ATV. However, this would still make the mission unreachable on foot.