I'm blind, but i bet the ghost icon doesn't show on my system, at least i can't see it with my screen reader.
Hard to say, as during the scan there is no indication that there was an error during scanning.
My list of the Scan History entries is loaded with them going back to May 2020. Looking in the Report folder (C:\ProgramData\AVAST Software\Avast\report\Full Virus Scan.txt) for one of my scans does show a file unable to be scanned.
C:\Users\xxxx_xxxx\OneDrive\Getting started with OneDrive.pdf [E] File is offline - it is currently not available. (42006) My guess this is obviously picked up by the other scans also. Since I don't use OneDrive I have deleted this .pdf file, so I guess that would remove the scan error from being reported and responsible for the Ghost icon.
EDIT: Confirmed, removal of problem file removed Ghost Icon from Scan History display.