How was the private investigator Kim Hyejin hired able to get those videos?
Earlier that day when Areum's manager ordered for a takeout, the private investigator disguised as a delivery lady to deliver the takeouts for him.
Areum's manager was in the shower when his door bell rang. He guessed it was his delivery and he quickly wiped his face and wrapped a towel around his waist before walking out from the shower and went straight to open his door.
Since he has already made a payment, he took his delivery and closed his door without checking to see if he closed it properly as he had to get back into the shower. The private investigator used that opportunity to hold the door just a little inch before it closed and waited for a few moments before walking in.