I was in the big factory in Cherno, found a backpack and wore it. Then went upstairs and found another backpack. When I right clicked the backpack and chose replace, I equiped the new one and the old one disappeared. I looked around and couldn't find it and decided to leave downstairs. I see the blue backpack again and place it inside my new backpack.

I tried the same thing in one of the cafes. Went upstairs, turned right, went inside the room, saw a backpack, repeated, replaced, the other one that was replaced disappeared. Went downstairs in the room below and there it was.