DavidR
7
If it is exploiting a vulnerability, then things can get past initially, especially when your on-access (standard shield) scanner is set to normal, which doesn’t scan newly created/modified files (a downloaded file, browsed or otherwise has to be stored on your hdd and as such is newly created).
However if a program or you tried to open it, then the on-access scanner scans it and alerts/stops it.
If however nothing attempts to open it, then it will sit dormant on your hdd until you start and on-demand scan (boot, local drives, folder or file). The on-demand scans all files you ask it to scan, so it works in a different way.
By having the standard shield set to high it will scan newly created files and alert if required. Unless of course you have your settings set to silent mode (in the Advanced tab of my previous image).