DifferentialReviewersView has this little visual flag for when an Owners Package reviewer doesn't own any paths in the visible diff:

if ($reviewer->isPackage()) {
  if (!$reviewer->getChangesets()) {
    $item->setNote(pht('(Owns No Changed Paths)'));
  }
}

This looks a little awkward in usage, and might be better as:

• No Paths

...or similar.