I wasn't really sure where to put this but I think I've stumbled onto something that may be the cause of your NPC's simply disappearing from your interior cells. It happens to me randomly when I enter the cell. Sometimes my NPC would be gone, but their package tells them to stay in their editor location. I thought they may be falling through the floor somehow so made a collision box spanning the room just below the floor. Then the NPC was gone again, and by chance just used TFC in the console, moved out a bit, and there he was, sat on my custom chair in mid air!!
http://i1159.photobucket.com/albums/p628/B1gBadDaddy/2013-02-09_00001.jpg
In conclusion, if you have disappearing NPC's, and their packages say they shouldn't, or aren't part of a template that may cause it, this may very well be a reason. NPC's will sandox anywhere within range on cell load. Proof! So remember to tick the Ignored By Sandbox option for your reference if it's not needed

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In case I'm the only one who didn't know this, I'm sorry for the spam
