"Ghost" pending update (backup_migrate)
OK I think I found it, although I haven't yet been able to confirm that the problem has been corrected, since all my modules are up to date.

Based on this question, here's what I did:


find the ID number for the pending update. In my case it was stated plainly in Drupal's update.php messages, so it's 7303. This information can also be found in the .install file in the module folder (e.g. function backup_migrate_update_7303).
access the database (e.g. via phpmyadmin) and go to the system table.
in this table, find the entry corresponding to the module whose update is still pending (in my case: backup_migrate)
edit the schema_version column for this row and give it the value of the update ID found in step 1 (e.g. 7303).


And that's it.