“...items will only show up if they are in a ship/entity that you either own or assigned to... ”
It doesn't appear that way. I have some owned items that either:
1) are owned by me, but located on another faction's ship,
in which case, I can see what room they're in, but not anything about what system they're in, whether they're in hyperspace or on a surface, none of that;
or 2) are owned by me, but worn by someone else,
in which case, I again see the ship and the room on the ship (and thus, where that character is), but nothing about the location. It may change when they leave the ship;
or 3) are owned by me, but dropped on a planet,
in which case, I see the actual grid coordinates.
So, at least with items you own, worn on another player or carried aboard another ship (whether your faction or not), you can see the ship's name, and the room on the ship. If carried onto the surface, it appears you then see more info.