"I forgot to show you this before. Look: proof that the corridor was empty once!"
And there on his mobile 'phone is a picture of an empty corridor. I notice the date.
"Is that right? You've not set this up as a lark have you? It was never that clear then."
"It was, it was! Just for the one day, mind. I had to take a photo because when I rang my wife to tell her she didn't believe me either."