I have six notes in Outlook 2010 that I use regularly. They simply vanished on Monday. Here are the relevant details:
Auto-Archive is turned on and runs regularly, but it's turned off for Outlook Notes.
There's no evidence of the missing notes in my archive PST file.
My Outlook Notes vanished in similar fashion about 6 months ago.
I'm using the Google Apps Sync (GAS) tool to sync my Google Apps account with Outlook.
I suspect GAS is the culprit. GAS was expanded to include Outlook Tasks, Notes, and the Address Book at some point last year. To the best of my recollection, my notes first vanished after these three features were added to GAS.