If I recall correctly, they are file signatures that OS X uses to identify and remove malware. Not sure when and how the files on your drive are checked (potentially right before they are opened?), but the XProtect.plist file is automatically updated by Apple, and that's what you're seeing.

The entry doesn't exactly mean you're infected, but just that your copy of the file was updated.

I have the entry for instance, but I was never infected. You can check your copy of XProtect with this command (I have 2076):

defaults read /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/XProtect.meta Version

Edit: Looks like you do get a message before launching an app, if it's identified by XProtect/File Quarantine.