I, too, get this message a lot. I do not understand its purpose since I can not respond to it by repairing or moving or anything else. I have researched this issue a few different times and I cant seem to find any definitive response on what it is; why it is; and what to do with it. Again, if it's not important, why does it occur? What can I do to make certain I don't see this message ever again, especially if it's pointless?
It isn't pointless, the scan report aside from any detections also shows files that can't be scanned and why they can't be scanned.
I don't know what you mean by you can't find a definitive answer, though this topic is unlikely to provide it as the streaming feature wasn't in older versions. But there are many such more recent topics that tells you why the entry is present.####
Those are are files in old virus definitions folders and avast is doing some housecleaning to keep the size used on the hard disk to a minimum. Avast now has streaming updates indicated by the _stream sub-folder name in the defs folder (in your changed subject title).
When there is a new virus definitions update, this includes the previous streaming updates so the old _stream updates are redundant and are cleared; this can happen if there is an avast update during a scan (not unusual), so the files are no longer present.
I feel that you are only seeing because you are probably doing frequent on-demand scans, something that really is unnecessary in a resident antivirus, the more frequent the scans the more likelihood you will come across this.