I believe it was after I got the 3rd key card - I made my way to the top left portion of the map.
There was one box in the room on the floor that I pushed all the way to the left against a small wall that looked like it had another (2nd) box on top of it.
I then jumped on box 1, and attempted to push box 2 to the left - off of the wall that it was sitting on.
As soon as I held the “up” button to start pushing, box 2 phased through the floor quickly - dissappearing. Afterwards, I tried respawning, but box 2 did not respawn.