Invisible Files (Quoted from Symantec)
Symantec AntiVirus 10.2 for Macintosh creates an invisible file named "SymAVQSFile " on any drive that it scans. The size of this file depends on the number of files on the scanned drive.
Symantec AntiVirus 10 for Macintosh creates two invisible files named "DiskQSF" and "DiskQSFLockFIle" on the installation volume. The size of these files depends on the number of drives scanned by the Mount Scanner.
Note: These files are not known to pose any problems. This information is provided to comply with all international laws requiring that users be advised of hidden (or invisible) files created on, or copied to, their hard disks.