Whenever you run the free Norton Security Scan, it removes all scheduled tasks in the root of Task Scheduler (it leaves the folders alone) , and it adds a scan task whether you have a scheduled scan checked in the settings or not. I've tested this in Win10 1607 and Win10 Pro 1903. This appears to be a serious bug that most casual users wouldn't notice.
Task properties:
"C:\Program Files\Norton Security Scan\Engine32\4.7.0.181\NSS.exe"
/admin action=run fmui="C:\Program Files\Norton Security Scan\Branding\Config.dll" mode=fmui ptkdata="C:\ProgramData\Norton\{397E31AA-0D78-4649-A01C-339D73A2ED35}\NSS_4.7.0.181" setup=CORE start=Scheduled