Dawson is a non-canonical character who was created for the film adaptation of The Incredible Theft in Season 1 of ITV's Agatha Christie's Poirot. He is the butler at the home of Tommy Mayfield and his wife Lady Margaret.
There is a scene of Dawson greeting Lady Carrington as she arrives at Mayfield's estate. He is also mentioned by name several times, notably by Tommy Mayfield. There is a scene of Dawson receiving a letter from the chauffeur of Mrs Vanderlyn with a request that it be posted. The actor who portrayed Dawson is not credited.