So in 2020/2021 I bought a book at either Target or Walmart, I read it and loved it, but it had a 2nd book/sequel to it, that I would love to find. The main issue is that I cannot find the original book or author, I cannot remember who wrote it or the title.

The book is based in the UK I think. The book was about a young girl (i think the public had nicknamed her Angel?? I can't remember) but she was found in a wardrobe/wall in this house by a female police officer who is trying to become a cop. The house was a previous crime scene where they found a man who had been tortured and murdered. They discover the girl because several people in the neighborhood report missing things, like food/clothes, and the kids recall seeing her. The cop spreads baby powder or something on the floor and waits until the next day and follows the footprints to the wall or whatever. They find the girl, but they put her in hiding in an orphanage-type place. ANYWAYS the book focuses on this psychologist guy who is invited to study the girl. After an incident, he decides he wants to foster/adopt her and does so. She moved in with him and through the story they sort of solve the murder of this girl together (kind of) and in the end the girl has to go back to the orphanage/home until she is 18 or something. I LOVED the book and want to re-read it and also find the sequel where the guy tries to look into WHY the girl was hiding in the wall, and why the dude had her in his house. It's assumed that maybe he saved her and was keeping her hidden and that is why he was killed. The cop who found her ended up leaving the force and hiding away too because of finding the girl.

Does anyone know this book??