My theory is that Kim Kangwoo's character had an affair with Lim Semi's character, Yuri. When Soohyun called them, they were having an affair(sex). Just as Soohyun returned home both her husband and Yuri got startled. Yuri hid somewhere and later, Soohyun entered the home. Somehow, Yuri escaped and she forgot to close the door, allowing Gunwoo to go outside.
Neighbor lady was taking pictures of her son riding a bicycle and she must have captured Soohyun opening and closing the door properly. I think these pictures will prove to Soohyun that she closed the door very properly, potentially revealing that someone else (Yuri) was at their house before she entered or maybe neighbor lady must have seen Yuri entering the house and she knew about Soohyun's husband's affair with Yuri thats why she was giving weird looks to Soohyun.
Regarding the accident, its clear that the main villain was behind it. Kwon Seonyul's father took the blame upon himself & now Seonyul want to take revenge on Soohyun.
Please share your possible theories also.
