I don’t know how to see those files. Maybe you can’t. They’re probably some system files that are in use. It might be completely benign. Hopefully someone else can give you more specifics.
It could also be password protected archives, like zip/rar/7-zip. The contents of those files would not be accessible to Avast without their passwords. If Avast doesn’t tell you what files it couldn’t scan, I’m not sure there is a solution.
Well you could manually scan subfolders on your drive and see what scan doesn’t complete or says it couldn’t scan everything. If that way of scanning even reports failed files. I don’t know. Maybe that’s a way to narrow it down. Just spitballing. Probably it’s nothing but if you have to know, wait for somebody from Avast or a user who knows more about the inner workings of Avast.