Some items that spawn in the back of the large convoy trucks are unable to be picked up, or seen in your vicinity. Here's an example of the ghillie rifle wrap not having an option to take (also doesn't appear in vicinity).
If it's relevant, this was from the convoy spawn near Tisy. Coords 2421/13603 approx.
That is because loot is spawned from a ground point height.
the truck is tilted but the loot spawn is flat to the ground based on the height settings in the proto file.
So much loot (Every cars seat that eats the loot you desperately need to survive) would be fixed if the reference was the tilt of the asset and not flatness of the ground.