So my theory is: Suho and Soohyun's sister were having an affair since the very beginning. She was in the house and left without Soohyun noticing. While leaving she left the door open. The neighbor saw her entering the house (hence she was feeling uncomfortable when she saw Soohyun returning to the house) and tried blackmailing Suho. They met at the hotel because Suho actually thought she could've sent the picture.
Funny how I can't get any depths of Na Hye Won as female lead character (they try to "justify" her actions by setting a problematic family - which failed miserably) but I simply fell in love with Kang Hee Joo's character with only a few scenes. That's the first time I'm getting a second lead (in this case, supporting act) syndrome with a female character.
Suho and Soohyun's sister were having an affair since the very beginning. She was in the house and left without Soohyun noticing. While leaving she left the door open.
The neighbor saw her entering the house (hence she was feeling uncomfortable when she saw Soohyun returning to the house) and tried blackmailing Suho. They met at the hotel because Suho actually thought she could've sent the picture.
Thank you so much for the recommendation, I'll totally check it out.