I just replayed through the first FIB mission, Dead Man Walking, because I'd missed the Unmarked challenge the first time round. In my replay, I found some fully protective armour, and then proceeded to get shot a dozen times, losing most of the armour. But in the mission summary I still earned Unmarked.
So does armour damage not count against the challenge, as long as you don't take any health damage? The description always led me to believe it did. What do you bros think?