AFAIK the path will be what the file path is at the time of detection, for example you have a thumb drive that was assigned to E:\, and Sophos detected a file there, the path will be something like E:\path\to\file.exe.
Yes - the file is probably in some network share, hence the UNC path. "Remote Files" under Real-time scanning is likely enabled that is why the remote file was scanned when it was accessed.