(To clarify, why would anyone think this girl is a boy? Absolutely not! I would never write such a nonsensical plot.)
Yu Liang would never actually peep on someone taking a bath.
He looked behind himself. The front door hasn't been closed yet, the corridor outside can be seen through the doorway.
It seemed unguarded and he could easily leave the girl's house.
But obviously, "Escaping the Locked Room" doesn't refer to this kind of uninteresting escape.
It seems like "he" chose to come to the girl's house on his own, so Yu Liang went ahead to close the girl's front door, he stood at the porch again to look at the girl's house.
The two rooms on his right should be the bedrooms of the girl and her parents. Between the two rooms, Yu Liang saw a table with a family photo on it.
Ahead was a small corridor, on the side was a bathroom, and at the end was another small room, probably the brother's room.
On his left was the living room, and further, the kitchen.
