βρήκα αυτό ..
Police Booking Procedure ....
"After an arrest, a criminal suspect is usually taken into police custody and "booked," or "processed." During booking, a police officer typically takes the criminal suspect's personal information; records information about the alleged crime; performs a record search of the suspect's criminal background; fingerprints, ..."
και αυτό ...
Booking Law and Legal Definition. In criminal law, booking refers to the process by which the police department registers and enters charges against a person believed to have violated the law.
So, I'm guessing that someone "in processing" is someone who's in the process of being "booked by the police."