When exploring Luna's house, you can contact mac on the tablet you get from the asylum
Text him until you ask him to come over, after the ghost has been captured Luna will be seen naked going upstairs
She will get in her room and ask you for some privacy, you can hear her and mac going at it but you can't see or interfere with the scene
HERE IS HOW TO GET PAST THIS HIDDEN SCENE.
Before the ghost is captured place a video camera in the room, when she is with mac you can watch them on the CCTV computer in the trailer
Apparently there is a second girl in the room which hints at the possibility of an orgy.