I heard some cracks behind the door of my stepdaughter's room when I was with her half-brother xv

I never understood the real reason for the noises I heard behind the bedroom door when my stepdaughter was inside with her half-brother. As I know that they have always had a lot of intimacy and have always been very reserved, I maintained all respect for the privacy of the two while they were alone inside the room, but even so, I did not fail to pay attention to everything that happened.