A client of mine has Outlook installed on his desktop PC and it's configured to retrieve emails from two providers. The messages get stored in separate folders. One of the email accounts is also configured inside a Gmail account so he can view his emails online, but Gmail is set to leave a copy of messages on the server.
What I noticed today was that several months worth of emails were missing from his Outlook. But doing a search in the Windows Vista explorer produced the missing emails (pressing F3 I think brought up the window). I tried searching for these in Outlook but none of his PST files contained them.
When I copied them from the Explorer windows to a new folder in Outlook, it said something like it was copying from a system folder.