Harry and Hermione excitedly reported to Dumbledore that they had been successful (saving Sirius, e.g.) during their time-traveling adventure in The Prisoner of Azkaban. In the movie, this occurs outside of the infirmary.
Dumbledore had moments earlier recommended that Hermione use the time-turner, before exiting the infirmary and closing the door. Why did Dumbledore feign to not understand what they were alluding to?
Is there something special about being inside the infirmary that makes it impossible to eavesdrop (at least according to the movie / screenwriters)?
Ostensibly, he would deny / repudiate that the time-turner was used / "abused" (in the eyes of the Ministry) for any other reason not formally proffered to Hermione (in studying; taking simultaneous classes). This is especially relevant in the event that such use explicitly contradicts the rulings of the Ministry (such as Buckbeak's execution).
Was the hallway just outside of the infirmary room a particularly more vulnerable location than inside of the room itself? After all, the door was wide open (at least in the movie) the entire time he was speaking and giving them advice ("time is dangerous when meddled with", "Sirius is located at...", "more than one innocent life can be saved", "3 turns should do it", "return before the last chime", etc). He all but explicitly spells out what they should do in gratuitous details, as those details were for them to figure out with Hermione's intelligence and Harry's resourcefulness. But the high-level idea of using the time-turner was his.
Why suggest them to time-travel and break Ministry rulings and then two seconds later refuse to recognize their achievement? Was it out of an abundance of caution? Did being inside the infirmary shield their conversation moments earlier? Was he afraid of being overheard in the hallway?
Or was being outside of the time-travel loop a reason somehow?
NOTE: I realize this is a deviation from the book. I'm asking about the movie version Dumbledore. What were the screenwriters trying to imply?