It may not detect it either, and you definitely need to use an AV as the first and primary layer of protection for your device. A second scanner is called a second scanner for a reason, just to ensure it's not being compromised.What if a zero-day malware sample missed by full AV (which has signatures and behavioral analysis), how is on-deman scanner will detect with only signatures?
I assume to get what slipped through an AV, I should use AV with more tools and has better implementation of those tools.
By the way, there are some cases when malware infects a device, and you can't install an antivirus at all because the malware prevents it from installing. In this case, some second-opinion scanners can be very useful, as they can remove the malware and allow you to install the antivirus afterward and perform a full scan.


