In the templates tab, I the constant COM_CMANDRILL_SEARCH_IN_TITLE displayed.
I can see this called in the administrator/components/com_cmandrill/views/templates/tmpl/default.php along with COM_MANDRILL_SEARCH_IN_TITLE="Search in title"
Looking in the administrator/language/en-GB/en-GB.com_cmandrill.ini, I think this is missing.
I see COM_MANDRILL_SEARCH_IN_TITLE="Search in title" which is used to display in the search field, so COM_CMANDRILL_SEARCH_IN_TITLE="Search in title" is missing as this is the label.
I added it to my lang file and it resolves the problem.