There is a subtle bug which was found recently in this area. I'm not 100% familiar with it (yet!).. but relates to swapping policies, and the Vault Cache or Offline Vault-ness of the user afterwards.
The hidden message can be seen with MFCMAPI, MDBVU32, or Outlook Spy. It's on the inbox I believe. Remove the evoffline one, and Vault Cache will disappear.