I'm not sure if this Achievement is missable, will update in the future if it is.
This achievement is obtained as soon as you open the lock to the Alleyway stash door.
Upon your playthrough of the We All Fall Down DLC, you may stumble upon this big metal door with a lock on it. You must simply pick up the key somewhat later in your playthrough, after you've completed the Haworthe labs sequence.
Upon completion of the Haworthe Labs sequence, the Waterloo district, (same district Haworrhe labs is in) will become night time, and will unlock a new area of the district.
Once you exit the lab, get past the chatting doctors and the Downer Detector checkpoint, then proceed through the thugs harassing the lady in the alley, you will come across a SECOND downer detector. Follow the yellow cord if you want to turn it off.
(just before you need to pass the SECOND downer detecter, look and veer left, and you can obtain the Canal stash related achievement after dispatching the plague wastrels in the canal, and proceeding through the tunnel and picking up the gadget parts. BTW I'm on mobile and I don't know how to cross reference achievements, sorry.)
After passing the SECOND Downer detecter, you will see a Lion statue and a Downer shocker TV to your left. Proceed between these and you will see the exit gate and its valve, with an alleyway to the right of it. Enter the alleyway. The key for the stash is found laying on the ground next to the corpse within the alleyway.
You may now head back and open 'er up, and the achievement should pop upon opening the gate.
Much credit for this solution, and thanks for clearing up my ever building anxiety for this guideless achievement goes to Y|yukichigai for a clear, concise, thorough WRITTEN guide, that told me exactly where the key was. If you were like me and were worried this achievement was missable, which I think it might be, amplified by the fact there is no guide for this achievement, I hope this all helps alleviate anxiety for anyone in the future. Bless ya, Y|yukichigai.
Link below.
https://steamcommunity.com/sharedfiles/filedetails/?id=21004...