I suppose this is a bug. I am doing the "Arniel's Endeavors" questline and got to the point where I am searching for a certain staff for Enthir (Staff of Tandil according to Wiki).
So Im following the marker thru Indella's Keep only to find that the spot the arrow is pointing to has nothing there!

Is there a way I can 'cheat' and just summon the darn thing somehow? Any other suggestions would be greatly appreciated!
Thanks!

I have exactly the same bug, although i have the marker point on a wall in Morvunskar where Sanguine just stands there doing nothing. It's the only quest that is bugged for me. I also would be pleased if anyone knew how to bypass this.

Try loading a save before you enter Indella's Keep. I've had this problem with other items before, and reloading that area has fixed it.

~ player.additem 0006a093 1

Badda Bing, Badda Boom.