As Brian mentioned this is defwatch scan - frequency for it cannot be set manually and is trigered by defs updated on clients:
-------------------
Run an Active Scan when new definitions arrive Starts an Active Scan when new definitions arrive to check for any risks that the new definitions can detect
By default, an Active Scan is run when new definitions arrive. If you disable this option, you weaken the protection available to your client computers. You should only disable this option if you have special configuration or exclusion needs that conflict with this automatically triggered scan.
This option is enabled by default.