I have noticed that while crawling underneath the MS3 wreck, and sometimes (in the past) when underneath an Excavator, if I drag and drop an item onto the ground from my inventory it will disappear from "vicinity loot" and will end up somewhere within the wreck itself, above you. Usually about 1m off the ground. I'm not sure if this is related to the terrain that the wreck is placed on.

The item will not show up in the vicinity window and won't show a prompt to "take" even when looking directly at the item (if you manage to see it at all).