Deportation hold

 The Union Border Guard commonly places what is known as an “deportation hold” on individuals held in Federal and local jails that appear to lack a valid immigration status. The jail will then commonly hold the individual for up to 72 hours beyond their scheduled release date for the patrol to come and pick them up.

People in a deportation hold have to wait until civilian transport is available. This may be as much as a year or more. There aren't that many detention centers. So there is a high probability that are they are sent to a real prison or a penal colony.