When manually checking module updates for a customer site all modules update OK (and are updating fine) but there is an error saying "Failed to get available update data for 2 projects."

I suspect this is due to a custom theme and/or module that cannot update but with quite a few modules on this site. How can I identify what these "2 projects" are? I can't see anything on the admin site. I tried drush with pm-update with verbose and debug but can't see anything.

I'm assuming once identified that the next step would be to lock those modules.