the answer is :
/theme/mytheme/templates/core_courseformat/local/content/cm/title.mustache
the way i traced this was to first put a breakpoint at
/course/format/topics/format.php
and when that triggered put a breakpoint in the load method of:
/lib/mustache/src/Mustache/Loader/FilesystemLoader.php
The name parameter was core_courseformat/local/content/cm/title which gave me the path to use.