I got this achievement when I joined someone trying to do the Challenge: The Founder King - The Mystic of the bonus mission and when I was at the camp before the mission started I got a prompt saying you have acquired a secret mission which was to make sure the host made it back to camp alive. If you skip past the instructions of what you're supposed to do because you accidentally pressed A on it you can hit A on the bonfire and go to mission briefing. According to MemoriesOfFinal you'll get the secret mission whenever you are playing with human players and won't appear if you are playing with AI.
Note: This achievement may not unlock the first time you do it. I had this happen to someone I was in a session with and we did a second mission where his challenge was to revive a teammate when he went down but he failed it and still got the achievement.