Looking more closely, there appears to be a 3rd sash that is movable from top to bottom.
During daylight hours (Obama and Trump), it is on the top and during nighttime (empty), it is on the bottom.
Think tossing or launching a grenade or incendiary.
Is this an added security device?
Well, if the worry is that during the day and the target sits at the desk, the nest sniper angle is from above.
At night, best angle of attack on a typically empty office is from ground level.
Ping.
A sliding sheet of ballistic glass or plastic.
Secondary protection.