EDIT #612: TU9 dropped on Apr 21 2020 and now the cheev is available, but it has changed.
You now have to access a safe house inside a DZ. Each DZ (South, East and West) has a safe house roughly in the middle of the map. Enter the safehouse and cheev will pop.
IGNORE EVERYTHING BELOW THIS LINE - IT'S NOT RELEVANT NOW...
EDIT: Since TU8 dropped 3/3/20, this might no longer be available because the Thieves' Den is gone. I am investigating...
EDIT#2: Looks like this is unobtainable right now
EDIT #3: Massive is looking to replace this achievement. No date yet. TL;DR
This is for entering the Thieves' Den. Hack SHD Terminals or pick chest locks until you find the location of the Den. Enter.Longer version
The 'Safe Room' is actually the Thieves' Den in the DZ.
As you wander around a DZ, you will see SHD Terminals here 'n' there on walls at eye level. If you walk up to one and hold Y, you will go rogue and the next objective shows up on the map.
Because you have not killed anyone, you are not a disavowed rogue, so you don't show up too much on the map.
Run to the next objective - it will either be a SHD Terminal or a Chest. Hold Y to hack a terminal or Hold Y to lock pick a chest.
Each time you hit one of these objectives, you're one step closer to triangulating the location of a Thieves Den. Once you hit 100%, three Thieves Den entrances show up on the map. Choose the safest location (i.e.; one that does not take you through a landmark) and once you get to the entrance you can enter.
Your rogue status is gone and you get some XP and currency. Make sure you pick up the shock ammo from the crate before you leave!
Sometimes the entrance is a manhole in the road or an out-of-the-way door.
PRO TIP: Steer clear of checkpoints - you are rogue, they have turrets, you will die.
EDIT: You can kick start this process by lock-picking a chest, too.
EDIT#2: You CANNOT do this in the Occupied DZ.
A SHD Terminal in DZ East https://imgur.com/c44gCQv
Location of the SHD Terminal above (circle, not the diamond) https://imgur.com/kcjsGzF
Another SHD Terminal https://imgur.com/mqUtTkU
Thieves Den is 100% triangulated and there're three options https://imgur.com/6noBrQ9
(The dodgy lookin' geezer is a vendor)