Already commented that it may have been under the metal siding or the roofing.
The I saw the trees by the ladder. Those trees could hide both.
How did the kid know that that particular building would not be guarded by law enforcement? That’s the big question. Also, where did the van with AZ plates come from?
The dude in the very first video, in the red Trump hat,said the kid was carrying the rifle as he climbed up the building right before the shooting.
How about in the backpack he had on.
You might want to check this interview out....lots of questions answered about the ladder and the officer who saw him on the roof.
So he knew where to hide the gun. Gee, I wonder how?