I had a similar thing to this last night. The scavenger was in the wall of the building. He could shoot out , and move. But only in the wall of the building and he was indestructible.
I think it's funny too, as this has happen to me, but I've also notice that when you, IF YOU, clear out a base a ship flies in with more, then more, then more, then more, and continued until I died and this mission was to collect an item and was just a level 1 ranked mission....LOL
I've had three variations on this "kill 10 people/scavengers" thing:
I am asked to kill 10 people, but the site (according to a comprehensive search in the SRV) only ever reveals nine people.
I am asked to kill 10 scavengers. There are actually 15 at the site, only nine of which actually count towards the score.
I am asked to kill 10 people, the tenth of which falls to the floor "dead" but is still treated (for scoring purposes), and stll shows on the SRV radar as alive.
All of those happened yesterday. Only the first one happened again today.
More than once I've run into a scavenger in a wall that can't be killed. Have to keep relogging until I get a batch of scavengers where none are wallhacking in order to complete the mission.