Early in the evening of July 4, Micaela Giles’s mobile phone started sounding alerts, and a series of messages straight out of a horror movie began scrolling down her screen. Her 19-year-old son told her that his Airbnb host in Madrid had locked him in the fourth-floor apartment where he was supposed to be staying and removed the key. The host was still there, he said, rattling knives around in the kitchen drawer and pressing him to submit to a sexual act. He begged his mother for help.