I sometimes find something on a wardrobe, usually a pants or sweater. But when i get close and check my "Vicinity" items there is no item.

It's not possible to see which item is on a wardrobe by checking items on "Vicinity" list. You can only pick it up by clicking F, IF you have space in inventory.

EDIT: I also noticed this happening on airport, behind the house where there is usually item on small cabinet-something that extends out of the wall. I wasn't able to pick up the helmet even by having space in inventory and clicking F.