BackendStatistics:
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UnknownFileSize: 12
UnknownFileCount: 3
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Is this something to worry about?
I don’t believe so.
I’d have to check the code but I think it’s just logging the count and total size of non-Duplicati related files found in the destination.
On a Windows touched destination this could include files like thumbs.db or on a Linux touched location .nomedia.
If you’re curious, try looking at your destination for files that don’t start with duplicati-…I suspect you’ll find 3 of them. 
It’s Google Drive backend. Is it possible to get Duplicati list the unknown files?
I’m not sure. Perhaps indirectly if you look at the Remote log you can see all the files returned be the destination but I don’t know if it will flag the unknown ones.
I’ve found the unknown files in the remote log. But these files was not found in the destination. I found out that the files that was deleted from the destination a long time ago and was left in the google drive trash. After removed them from the trash, no unknown files is listed in the log.
Interesting discovery! I’ll have to see if I can get the same result with a deleted Google Drive file…
Since you resolved the issue yourself I went ahead and flagged your post as the solution. 