MMF2, but I'm wondering it's something that Windows is doing in the background. I've disabled my network connections (going off the error I thought someone might have acces to the file).
It's random so I can't replicate it but it only happens on one MFA document.
I've had this bug happen before on vista with .mod music files.
It happens at total random, but sometimes windows just simply takes control of a random file and won't let you delete or modify it. I had to finally reformat to get rid of the problem and haven't had it since.