Per ZoneXbez on Twitter, who has been hunting to solve this side quest for a few days, the safe is indeed behind a painting in Richtofen’s office. Mr. Peeks is required to remove the painting from the wall. Inside the safe is an Aether blade and some Intel.
(someone cheated the Mr. Peeks field upgrade into the game and was able to do it)
Source:
Leave it to modern day Treyarch to tease a side Easter egg that isn’t even completable until some time after the map has released—drip feeding content at an all time low.
Edit, to confirm steps:
Obtain the Safe Key from Dark Entity Containment. Locate the three paintings hidden around the map (Assembly, Sublevel 10, Reception (behind door in portal room) and go place them in the Director's Office, each one will drop a 500 essence drop when placed. Once all three are placed, use Mr. Peeks near the painting behind Richtofen's desk. This starts a small lockdown against an HVT Disciple. Kill the Disciple, and the Intel and Aether Blade will be available to collect.