Seen it a few times, and finally figured out it was Eddy I was seeing through my lock screen. Occasionally if I install/remove an app through Eddy and leave it open. When I return after a lock screen, there will be a weird square showing through the lock screen. When I close each window it shows up as Eddy. And once focused the shaded box goes away and shows the actual Eddy window. No other Eddy screens seem to show this.
I am not sure if this is an Eddy issue or a lock screen issue in popos.
Not sure of the eddy version I am using
PopOS 21.04
Kernel 5.13.0-7614
Gnome 3.38.4
Not sure if you need anything else.