The answer to this question is that a cached version of the webpage has been viewed, and due to the way that the Site Search Settings are configured in Google Analytics, this pageview is being counted as a site search.
When you view the cached version of a page from Google's SERPs, the URL is appended with something like: https://webcache.googleusercontent.com/search?q=cache:RVAzSsB6xI0J:www.example.com
The Site Search Settings on the website I was dealing with had been set up to regard the site search query parameter as 'q', meaning that whenever 'q' was detected in a URL, any subsequent characters would be considered to be to be an site search query.
I am going to rectify this by changing the query parameter to be more specific (whilst still matching the correct URL structure) to ensure that only true site searches are counted and reported in Google Analytics.