Maintenance page being rendered as an entire page load after the main page load
I'm still waiting to see an answer to the question of what causes maintenance mode to be displaying messages. So I don't know specifically whether the recoverable fatal error (this issue) being thrown by the buggy 7.x-2.4 version of XML Sitemap module was what caused the avoidance of a general WSOD/ crash and yet a display of an odd maintenance mode message. 

I had not noticed they'd released 7.x-2.5 and so I've updated that which has stopped the behaviour altogether. Happy days :)