Hello,
The Virus Scan run on the assigned Policies.
So, Incase, if the machine has a Managed Client, however due to some reason not connected to network would have the Antivirus / Antispyware Policy to Run the Scheduled Administrator Defined Scan.
Again, AutoProtect Scan always Protects the Files on the machine.
However, incase this machine in question have a Self Managed SEP 12.1 Client, then it has by default Active Scan upon Startup and Scheduled Active Scan.
Again, you can always create a Scheduled Full Scan as per your requirement.\
To know more, I would suggest a look at the Solved Comment in the Thread below:
https://www-secure.symantec.com/connect/forums/trojan-virus-only-found-scan
Hope that helps!!!