Launchpad parses bug comments for links to other bug tracking systems and identifies these as bug watches in the “Remote bug watches” portlet. However, bugs with upstream tasks that are linked to bug reports at other bug tracking systems are also referred to as bug watches. So a report like the Debian bug tracker bug watches isn’t useful for finding these unlinked watches because it contains both kinds of watches.
Subsequently, I wrote a database query to find all the bug reports that have comments referring to another bug tracker but no upstream task. The subsequent report contains the bug number and the name of the bug tracker mentioned. It only includes Ubuntu bug reports that are New, Incomplete, Confirmed or Triaged and is updated daily.
What do you do if the remote bug mentioned in a comment is not related? It is possible to delete these bug watches by clicking on the pencil in the “Remote bug watches” portlet for the relevant bug watch and then clicking the “Delete Bug Watch” button. This will remove them from the next run of the report.
There are currently 1629 bugs listed in the report, but ideally the number should be close to zero.