[Dovecot] dovecot.index disappeared

[Metro Domain Admin]

Hello,

One of my users just mysteriously lost all files from his inbox. I have 
"mail_log_events = delete undelete expunge copy mailbox_delete
mailbox_rename" but couldn't find any evidence of accidental file or
folder deletion. I logged in to find that
~user/mdbox/mailboxes/INBOX/dbox-Mails/dovecot.index and its backup were
tiny, only 552 bytes, when the folder had thousands of messages. Other
folders were left intact. We are running 2.1.15 (mdbox) on Ubuntu 12.10
using ext4. The system was rebooted about 10 days before and the full
INBOX was definitely seen since the reboot. I'm just looking for ideas
about what might have happened.

Thanks,
Tony