I'd suspect malware or other. Since you can fix it with attrib why not put that in a script for easy access?
Also, what are you scanning this PC with?
I have a folder that's refusing to remain visible. This folder is located on a drive other than the system drive and primarily contains photography. It's generally only accessed through Explorer or Adobe Photoshop or Bridge. This folder has acted completely normally for months. Over the past few days it, seemingly randomly, has been having the Archive, System, and Hidden attributes turned on. When I use attrib to remove these they reappear with in 5 mins to 1 day or so. I am running the latest updated version of Kaspersky Anti-Virus and it tells me that there are no active or quarantined threats. Does anyone know of something that could be resetting these attributes that is not a virus?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic