Hostages show up in many quests, but there is one that will net you 3 per quick run.
You will need to progress the Inhuman quest chain until you unlock the mission "Day of the Remains."
-Start the mission and run to the first objective. This gives you a lot of XP for just getting here.
-Go to the next objective which ends up being 2 hostages to rescue for this achievement.
-Between these hostages is a SHIELD cache that opens up automatically. It has 3 chests. One of which is a cache strongbox for another achievement.
-The enemies in this area are optional. Kill them if you need daily challenge progress or XP, or just run past them.
-Next you will go towards the next objective, but go a little to the left. You will see a depot there. There are 2 chests outside of the depot, and 2 inside. You will need a hacker character for the top chest and anyone can open the bottom one which is always a gold chest. Opening 100 chests will get you this achievement:
and randomly these chests will award you intelligence towards this achievement:
Breaking into the Depot counts towards this achievement:
-After the depot, face away from it and go to the right. You will see another question mark location. Here is another hostage and a chest right next to them.
-At this point you may pause the game and return to the Quinjet with Y and repeat the mission as many times as you need.