I think you did a good job of making the garage look trashed, and you have a lot of items as evidence of making the robot.
I agree that it's confusing why one hand was left behind.
I noticed the boy on top of the robot (plus the huge hand), so I think the scale is clear. But it wouldn't hurt to add some trees, too.
I didn't notice the footprints at all (inside or outside). But now that you point them out, it seems off. The legs & hips of the robot (what we can see) are about the same thickness as the arms. The footprints on the hill seem only slightly bigger, but the footprint in the garage seems larger than the ones outside. Also, why is the foot just a circle when the hand is so detailed? My suggestion is to just take out the footprints. Although, you can leave the one in the garage, since it just looks like part of the mess to me, and not left by anything in particular.
I don't think you need to worry about the debris. Whether or not the roof is intact doesn't seem very important to me. I would make sure that the inside of the garage is darker than outside; to emphasize that we are inside a building, since you can only barely see the edges of the walls.