CREDIT
https://www.gamesradar.com/the-divisi...

At Washington Monument Down the rope and to the monitor at the back of the basement. Interact with the button and a zoomed in map of the surrounding area will appear with three locations circled. These are the locations of three agent graves.

The Division 2 masks - Grave locations

You simply need to visit all three at night time and salute them, while facing in the direction of the Washington Monument. You'll know that it's been done correctly because your UI will receive a staticky effect. When you've done all three, head back to the Washington Monument.

The Division 2 - Ghost Mask

Go back down the rope to the monitor and interact with the button again. An image of an orange circle will appear on the screen. Go back up the rope and outside, where you'll see an orange circle highlighted on the ground. Stand inside it for a few seconds and the Ghost hunter will appear directly in front of you. Kill him and congratulations! You've earned the 12th and final hunter mask, along with the eighth Ivory Key.