ClamXav Sentry is not available in the Mac App Store version of ClamXav.
ClamXav Sentry provides the ability to watch folders for changes to their contents.
As soon as a file or folder is added to one of the watched folders, it is scanned by ClamXav.
If the new addition is clean, no further action is required on your part.
On the other hand, if it is infected, a warning message is displayed immediately alerting you to the fact and allowing you to decide which course of action to take.