That I have seen happen in the Ysgramors tomb. I just closed draugr inventory and pretended I never saw that. But anyway it may be considered as glitch. Those draugrs has random armor part in them. Instead of having unique leveled item they have the common one which is used in many containers in the game and that one contains imperial armor. But don't worry you can easily fix it. Just open Skyrim in CK, remove imperial armor from the specific leveled item, rename it's IP so you'll create a new one and then replace the old leveled item with that new leveled item in the draugrs. That solves your problem if you are on PC - but if you play on consoles then you just need to pretend you never saw the imperial armor there.