I have done a bit of homework, and the facility that I described applies only to certain files as noted below which are liable to be changed following Windows updates/patches and which change makes AVG suspicious.
kernel32.dll Change C:\WINDOWS\system32\kernel32.dll
user32.dll Change C:\WINDOWS\system32\user32.dll
shell32.dll Change C:\WINDOWS\system32\shell32.dll
ntoskrnl.exe Change C:\WINDOWS\system32\ntoskrnl.exe
So it is not a general "exclude" function.
I also mentioned the facility was enabled by clicking the F3 key whilst the scan result window was open. Another error on my part, it is when the Test Centre window is open
Please accept my apologies for leading you astray. It was not my intention to do so, I am sorry for the confusion that I caused.