Is this is a upgrade from previous version where you had a policy for daily scans?
Sometimes when you upgrade few registry entries do not get deleted most of the times due to permissions issue.
As every polci yyou make registry vlaue is created for it.
So it might be a case that a old scan policy is running with new scan policy.
Check this registry location on any client and check if you see any addtion scan running
HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\Symantec Endpoint Protection\AV\LocalScans\
The scan name would be something like this :5df13630-79f7-4c70-002b-16b8952f5533
and it gives the details when it wil run and what type of scan it is.