This happened today morning. (surprisingly this is very similar to Steve's post!)
Some of our exterior walls started vanishing after our users saved to central. It looks like the exterior walls were deleted by mistake and the mistake was saved to central just before end of day yesterday. Today morning people opened their local files and saved to central, only to find that some exterior walls vanishing. We didnt want to rollback or use backups because so much of work has already been done. Even though it would have been easier to create the missing exterior walls, the original ones had inserts (which were scheduled), dims, etc associated with them.
We worked around by opening a local copy (that was not yet saved to central yet) and copied and pasted the missing elements into the central file. Subsequent saves to centrals after the missing elements were added into the central file turned out ok.
Now, how did the walls get deleted in the first place...?