Pearse, Sarah. Detective Elin Warner series #3

After the dark events that scarred her childhood, Kier Templer escaped her hometown and twin to live her life on the road. They've never lost contact until, on a trip to a Portuguese national park, Kier vanishes without a trace. Detective Elin Warner arrives in the same park ready to immerse herself in its vast wilderness - only to hear about Kier's disappearance, and discover a disturbing map she left behind. The few strangers at the isolated camp close ranks against her questions, and the park's wild beauty starts to turn sinister. Elin must untangle the clues to find out what really happened to Kier. But when you follow a trail, you have to be careful to watch your back