Bug Exterior Models Not Showing
The Bug
This is a bug you will encounter if you are following the Lokir's Tomb tutorial, specifically this step. You will also encounter this anytime you place a large static in an exterior cell such as a farm house or ship. When you place the pieces in the editor, they will show just fine.
But when you get into game, it will probably be gone. And then it might come back. Then disappear again.
Temporary Solution
Double click on the object in question to open its Ref Edit box, and check the flag "Is Full LOD" This is a temporary work-around not to be used as a permanent solution, as having too many objects flagged this way can severely impact performance.
Notes
Oddly enough, I had to add in a new static to show off the bug. I had already marked my tower as Full LOD, and toggling it off didn't cause the bug to happen again. More testing required!