There is a highlight button the ｽ key above the tab key, not sure if it highlights everything though.
As far as I can tell that key with the tilde, `/~, only highlights your characters and maybe other npcs (I am still on beach so haven't encountered other people to check it with yet). It seems like some loose items on the ground can be displayed with alt but I think containers and other lootable items are not (seems like a poor design decision).
