Poirot, Miss Oliver and Supt. Wheeler walk around the memorial discussing the case. Later on Poirot and Colonel Hughes do the same in episode 10x02 "Cards on the Table".
From episode 8x12 "My Mother, My Chaperone/The Present/The Death and Life of Sir Albert Demerest/Welcome Aboard: Part 2".
From episode 6x07.
From 2013 Christmas special "The London Season".
From episode 1x01.
From episode 1x03 "Having It All".