Federal Surveillance Center K-21B

This location is initially marked as Abandoned Shack in the map. This lone cabin is found in the map boundary to the SW of the Commonwealth, in the midst of The Glowing Sea. There's an armor workstation outside Loot the Installation K-21B key from the suitcase. The entrance to the actual installation is on the floor, literally a few inches away from the skeleton.

Inside the area, there's a personal log holotape beside the typewriter. Continue down the hallway then activate the circuit breaker. Continue inside and prepare to encounter some synths. There are only three of them inhabiting the place so you can freely explore the area after defeating them.

At the bottom floor, there's a Power armor to the east. There's also a mini-nuke under the stairs beside the power armor.

There's also a trunk in the opposite corner that contains ammo. Beside the trunk is a Novice mainframe terminal and Advanced safe. If you successfully hacked this terminal, you can reboot the mainframe and reactivate the elevator and deactivate the sentry robots (these are automatically hostile towards you)

Beside the mainframe terminal is an issue of U.S. Covert Operations Manual.

Exit using the elevator and the Abandoned Shack and its icon will be replaced by Federal Surveillance Center K-21B.