The last time he Plummer, speaks to Landau
...as he's (Plummer) listening to Landau on the phone in the hotel, he turns around and stares at the back of the door - the camera has him looking very intently at the door. On the back of the door there is an 'floor plan' for stairways, exits etc..
That is the start of Plummer realizing he's remembering similar 'plans' at Auschwitz - he stares and stares at that 'floor' plan and it remiods hm of the same 'camp plan'