Welcome to the forums,
Vote for these RFEs:
Bug 27005 - Prompt for password in offline client (for zimbra/non zimbra accounts) Bug 14713 - encrypt offline messages
Based on feedback I've seen,
even though you can still find/search the store folder on disk, some want
RFE 27005 for pass now anyways - as someone dropping by and navigating the store isn't as 'easy' as opening Zimbra Desktop.
(It's really a false sense of security though.)
Current best practice is to set a OS level password (login/screensaver & not share accounts), OS encryption or storage/disk encryption.
See also:
Bug 27936 - offline : Multiple Zimbra Desktop users on one machine Bug 15608 - install application vs data location