|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really existed), and, as such, details contained in this article may differ from real world facts.
At some point, there were Muggle complaints of 'wailing sounds' coming from plug holes and toilets; around the same time, wizard Philbert Chivers went missing and his wand was found at a sewer entrance. Suspecting there was some sort of magical creature in the sewers, Muggle Liaison Officer Myra Curio dispatched Department for the Regulation and Control of Magical Creatures investigators Mathilda Grimblehawk and her partner to look into the matter.