Corrected entry: At the end, when Sefton has unmasked Price as the spy, he reaches into Price's coat and pulls out the black queen, stating "The one you pulled out from the corner of your bunk and put in this pocket!" When the camera was looking at Price pulling the queen out, Sefton was on his back looking in the direction of his feet, which were pointed at the wall next to the window. So he couldn't have witnessed the event.
Movie Nut
20th Jun 2014
Correction: By that point Sefton knows Price is the spy. It's reasonable to assume he'd investigate and learn where the hollow queen is.