The shriek of whistles and a cacophony of cars honking have taken on new meaning on the streets of major U.S. cities in recent months — a warning to all those in earshot that immigration enforcement is nearby. The warning tactics from activists have become popular in cities recently targeted by Border Patrol immigration operations. Jill Garvey, a co-director of the pro-democracy group States at the Core, said during the first nationwide virtual “ICE WATCH” training session hosted with the help of community organizers from Chicago on Nov. 21. More than 500 people attended the online training session from dozens...