On pg 27 of the Virtual Machine integration Guide:
During a vStorage Image backup, Data Protector creates a snapshot and copies data from the
consistent state to Data Protector media. Then Data Protector deletes the snapshot
Did you have a VEPA backup fail? this may have left the snapshot behind. I found a lab case for DP 7.01 that describes this problem which ocurred only when using Change Block Tracking, but it was fixed with a VEPA patch
Also, if the snapshot was created manually, and not by Data Protector, it will not be deleted
One other note:
A full backup in the Single mode progresses in the same way as a full backup in the Disabled
mode, with the exception that the DP snapshot is not removed at the end (you end up with one DP
snapshot).