I have a vSphere 5.0 VM that was created with entirely thin-provisioned disks (I don't have any other options when creating them). A year later, one of the disks is now no longer thin provisioned but is Thick Provision Eager Zeroed.
I'm not licensed for storage vMotion (I do have normal vMotion), it hasn't ever been migrated between datastores via offline migration and as far as I'm aware nobody has manually inflated VMDKs.
I do know that at one point the filesystem on the disk in question became full; would this have triggered a conversion to thick provisioned?