I'm running Zimbra 8.0.2 FOSS and am experiencing a puzzling issue. Within the past week or so, two users have reported that their inbox has become empty, without any action of their own. The first incident was around 3pm on May 1st, and the second incident was this morning (May 8) around 8:30am.

Upon investigation, it looks like the contents of their inbox has been moved to the Trash. However, any folders they have nested within their inbox seem to be untouched.

With both users insisting they didn't (knowingly) do anything, how can I troubleshoot this to determine if it's a Zimbra problem or not? How can a user accidentally dump the contents of their inbox into the trash without knowing it?

Could another device (smartphone, tablet, or mail client) be a culprit?

Are there any logs that may show these events (when the messages were moved to the trash)?

Any help/insight would be MUCH appreciated!