Have you enable auto protection ?
Auto-Protect is the first line of defense against threats by providing real-time protection for your computer. Whenever you access, copy, save, move, open or close a file, Auto-Protect scans the file to ensure that a threat has not attached itself. By default, it loads when you start your computer to guard against threats and security risks. It also monitors your computer for any activity that might indicate the presence of a threat or security risk. Auto-Protect can determine a file's type even when a threat changes the file's extension.
https://support.symantec.com/en_US/article.TECH94990.html
Configuring File System Auto-Protect for Windows clients
https://support.symantec.com/en_US/article.HOWTO27140.html
You can scan also run Threat Analysis Scan tool
https://support.symantec.com/en_US/article.TECH215519.html
https://support.symantec.com/en_US/article.TECH215550.html