Permissions for random users lost out of nowhere

This has happened to me a handful of times since implementing SharePoint 2010.  Yesterday a user had full access to his site as a Site Owner.  Today, his permissions were gone.  Even my own permissions were gone.  I mean every permission for our usernames that was assigned to groups and to unique areas were removed,  everywhere. 

The Audit log does not show any permissions changes regarding this.  I had done Windows Updates on the SharePoint and SQL servers last night and rebooted them, and a full nightly backup happened last night.  My intial thought before when this happened was that the SP/SQL servers were rebooted when someone was in SharePoint files or sites, cause them to somehow get their permissions wiped clean, but I've had no trail to follow.

I suspected something with User Profile Service Application and the profiles not synching, but that sync process is working fine.  I have to manually reset the permissions for my account (which is in the Owners group of all the sites, the Site Collection Administrators group, etc.), and for this other user, with no understanding of what caused it and no known way of restoring all the permissions at once (tried a full synchronization of the user profiles), even the unique permissions that would have been set on specific libraries/files/folders.

This has happened with random users, about 5 times now.  I'm on the latest CU (Feb 2012).  I'm not sure where to look for the cause...has anyone else experienced this?