First a warning. The tracker is a bit buggy in a couple of ways. It either does not go up, or it goes up by 4%. That means if it counts, you need 25 cache strongboxes and not 50. I've had it go up 4% several times in a row, so it's not a matter of it going up every 2 strongboxes.
A cache strongbox is a loot chest that spawns in a SHIELD cache (bunker hidden underground). These cache locations usually have 2-3 chests in them, but you want the one with a little icon over it. That chest will provide you with a vault mission location and progress for this achievement.
Stark Realities is one of the first quests you will find these in, but there is a better mission as it is fast to collect and has many other achievement opportunities in it.
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. If you do not need any hostages, you can ignore them and just go into the cache.
-At this point you may pause the game and return to the Quinjet if you do not want to work towards more achievements. If you want more achievements, keep reading!
-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.