MIRAMAR, Fla. — For years, a coalition of clergy, volunteers and immigrant-rights organizers has kept a standing vigil outside the Broward County field office of Immigration and Customs Enforcement, a squat administrative building off Southwest 145th Avenue where people are supposed to check in on their cases and leave the same day. On July 1, that coalition gathered again — this time to say the building had quietly turned into something else: a holding site where people wait for days in crowded rooms, with little food, water or sleep. Barely 24 hours later, a member of Congress let herself in to see for certain.