Sean Donis’s wife, Nancy Donis, 38, said she was going to dinner. Donis stayed behind to watch their 5-year-old son. When he couldn’t find his iPad, he turned on the Find My iPhone app to locate it. The software showed the electronic device moving toward an unknown location; he had a hunch that his wife had taken it, and he decided to follow. He arrived at a house and opened the unlocked door. On the second floor, he found his wife in bed with her boss, Albert Lopez, 58. With his iPhone, he recorded two brief videos of them in...