It's safe to say that the search built into WordPress isn't exactly the best. It serves to a point but can't handle complex searches.
I was indicating handwritten posts by inserting (By pen) at the end but, due to the limitations, this isn't searchable. I've, therefore, decided to change it to a hashtag which is automatically converted to a search link - #bypen.
This serves no real purpose beyond it being interesting to see how many posts begin on paper and whether that behaviour changes over time.