The most likely explanation is a server glitch.
Why does everyone always blame the "computer" equipment? 
Because in this instance it's the most likely explanation. For posts to have gone missing,
and for there to be no record of those posts in the server logs suggests that the MySql database was involved, since it's that database that records and stores references to all the postings. Now, professionally hosted Apache servers normally use industrial-strength uninterruptable power supplies to avoid this very problem but, in certain situations, such as severe magnetic storms at or near to the server host, glitches can and do occur.
Any manual deletion of the posts would have been revealed through the logs and we haven't been hacked, because we'd know about that pretty quickly, so I'm going with glitches on the database server.