I know this might sound stupid but it may likely be from a mod installed. I've played Fallout 3 for about five years on both 360 and PC and never encountered this issue so that's what makes me think it is the result of a mod. What I would do at this point would be to create a copy of your save game, save your load order, and then try troubleshooting the problem. I've ran into a few mods that caused unnecessary changes in the past. (i.e. Fort Exodus)