The Boy in the Suitcase by Lene Kaaberbol

From Publishers Weekly: "In this riveting first in a new Danish crime series, hard-working Copenhagen nurse Nina Borg can't say no to nursing school friend Karin's cryptic request to pick up a package at the train station. There, stuffed in a suitcase inside a locker, Nina finds a three-year-old boy, drugged but alive. A near altercation with a violent man, who arrives at the locker soon after and is furious to find the suitcase empty, quashes Nina's instincts to call the police. Child in tow, she tries to track down Karin to understand her involvement and discover whether the boy, Mikas, who speaks only Lithuanian, is a victim of sex trafficking. Meanwhile, others are searching frantically for Mikas, from his mother in Vilnius to the men who'll stop at nothing to recover their "cargo."