Jabber,
We looked through the code, and that's a minor bug on our part that you can safely ignore. When you enable the Find Version setting, if a file is found to be modified on your site, VVT will try to scan each file from previous versions to see if it matches the file that is different in your version.
For some reason, it sounds like the file that is modified on your site doesn't exist in the Joomla 1.5.15 or 1.5.16 versions. Can you tell us what file it is that's being reported as modified? That will help us narrow down our theory and correct the issue if so.
Again though, it's a PHP notice (which is just to let you know something not-normal happened), but not something critical you should work about.
Thanks,
Alex