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
The best use of the "Is Full LOD" flag is for testing purposes only during mod development; unchecking the flag before final release. There is significant evidence that the bug is not carried over to the user end, that it only affects the plugin author, not the players, so leaving it checked for players is not necessary given the cited risks of performance loss.