Is it normal to have a hidden file named .file in /? I searched google and found a list of hidden directories and files in OS X at http://www.westwind.com/reference/OS-X/invisibles.html
but .file wasn't listed.  The file is 0 size and looks like this

----------   1 root  admin        0 Aug 24  2013 .file


very suspicious?

Running McAfee virus scan there were over 700 additional files that were Not Scanned because they were protected. 


/private/var  
/private/etc  
/Library/Caches  
/Library/Application Support
and others
You can query the package receipt database.

pkgutil --file-info /.file


on a machine running OS X 10.6 the results are

volume: /
path: /.file

pkgid: com.apple.pkg.BaseSystem
pkg-version: 10.6.0.1.1.1249291854
install-time: 1252683091
uid: 0
gid: 80
mode: 0

pkgid: com.apple.pkg.Essentials
pkg-version: 10.6.0.1.1.1249291854
install-time: 1252683099
uid: 0
gid: 80
mode: 0