Admiral Ponsonby is a non-canonical character created for the 1987 adaptation of the novel At Bertram's Hotel, as part of the television series Miss Marple. Selina Hazy thought that she saw the admiral at the hotel, but it turned out that it was just someone who rather looked like him.
The cause of the mistaken identity is possibly due to the fact that the criminals use the likeness of well known faces as part of their criminal activities.
Admiral Ponsonby is probably the same character as General Arlington in the original novel.