Security Base is one of the trickier maps in the pool, with quite a few locations that can confuse newer players. There may still be a few spots missing, but these are the ones I personally know about:

Starting on the first floor of the main building, this check is inside the door on the left, on the back wall as you enter.

This is inside the door adjacent to the previous.

Upstairs, this one is just outside the window.

From the previous, you can check this location easily. This is at the back of the map, at the rear entrance.

Climb the ladder to the left of the previous, and this check is on the roof.

From the roof, going all the way toward the previous location and looking over the railing, you can look down on this location. This is the alleyway along the left side of the main building.

This check is the final location in the main building. This is downstairs in the holding cells.

Facing toward the main building from the truck, this building is the one to the right, closer to the center of the map.

Inside the smaller building adjacent to the truck, this check is two doors deep; the first two that you'll come across after entering.

Downstairs in the smaller building, this is the only door to check at the back of the hallway. In MPO+, this door will be unlocked. The check will be at the back of the room, past some stacked crates.