The Adventure of the Retired Colourman

Sherlock Holmes is hired by a retired art supply dealer from Lewisham, Josiah Amberley, to look into his young wife's disappearance. She has left with a neighbour, Dr Ray Ernest, taking a sizeable quantity of cash and securities. Amberley wants the two tracked down.
Holmes is too busy with another case at the moment, so he sends Dr Watson to Lewisham to observe what he can, although Watson is keenly aware that this is more Holmes's province. He does his best, observing that Amberley is busy painting his house, which seems a bit odd. He even sees Amberley's wife's unused theatre ticket; she and her young man disappeared while Amberley went to the theatre alone after his wife complained of a headache. Watson notes the seat number.
Watson also sees Amberley's strongroom from which his wife had taken the valuables. She, apparently, had a key of her own. He meets a lounger with a rather military appearance in the street, and later observes him running to catch the train at Blackheath Station as he is returning to 221B Baker Street. Holmes recognises the description; it is his rival in detection, Barker. It later turns out that Ray Ernest's family has hired him to find the missing doctor.