This check is up the ladder on the side of the room you'd come in from in the base game.

From the previous, you can see both of these checks opposite.

This check is in the security office overlooking the main room with the previous checks.

These two checks are in the machine room, accessible through the side door down the hallway where the Mosin-Nagant is in the base game.

This check is on the lower level of the room adjacent to the machine room.

And this one is on the upper level on the balcony overlooking the same room.