I do not think it is a part...I believe it is packing foam for the actual part that is a small cover. Number 1K0-868-887 is for many small plastic covers and is usually accompanied by four other numbers and letters such as; D-82V, D-3U6, D-71N etc. Perhaps a small cover was found damaged or lost by the repairer and they ordered a new one and missed it upon final clean up. You can do some online searching using 1K0-868-887 followed by those other four I provided. Perhaps one od the covers will be familiar to you that you see in the cargo area.
I am getting nothing for just 1k0 867 887. Everything I see is 1k0 867 887 xxxx, and is a plastic cover of some sort. Nothing that is styrofoam. Can you go back to the repair shop and ask the tech that actually worked on it?