Change Receipt Email Title: Template, Localisation File or Somewhere Else?
Normally this would be done in the System Workflow message templates - you can change both the message subject and text there, as well as adding more extensive customization using Smarty tags. So it's odd that it's not showing there...

One other option I can think of is that you can change certain pieces of text globally using the Word Replacements feature (Administer > Customize Data &amp; Screens > Word Replacements. So perhaps if there was a word or phrase that you changed not only for the receipt titles but anywhere it was used on the site, you might find it there?