Isn't this related to the Daedra artifact found in the college basemant?
No, I believe you're confusing your invisible chests!

http://supremegamer.hubpages.com/hub/skyrim-midden-daedric-relic-puzzle
The one on that island is from the College of Winterhold The Midden quest where you encounter the daedra Velehk Sain. He can point you to that invisible / non-existent chest.
Whereas the one in Dawnstar is considered an easter egg. Could very well be a part of a broken quest or a quest that was wholly cut from the game in the last minute.