Would a scan detect something that On-Access wouldn’t and how does one know if On-Access is working properly? (I know you can determine if the scan is working correctly.)
Thanks
VFN
hi VFN, welcome to the forum 
I think I am right in saying, a scan would find something that may not have been activated, so for example a file that has been stored away somewhere on the pc and not run.
On-access means that files are scanned when run/opened etc.
To test wether the on-access scanning is working, you could take a look at the eicar test file:
http://eicar.org/anti_virus_test_file.htm
A harmless file used to test whether an AV is working correctly.
-Scott-
I think if there is a virus inside an archive I am giving to a friend,
on access need not to catch it, not being accessed.
AFAIK on access does not perform thorough analysis within archive, as on demand scan can do.
It would be performance loss.