In most jurisdictions, an arrest warrant is required for misdemeanors that do not occur within view of a police officer. However, as long as police have the necessary probable cause, a warrant is usually not needed to arrest someone suspected of a felony.
let's say the police spots a van full of people or a house.. since the majority of legal residents don't drive in overcrowded vans or on top of the pick-ups, don't live in over-crowded houses, there is a resonable suspicion to believe those people have broken the federal law and are staying in the country illegally.. if it's not so, they will not be detained or arrested..