This does sound rather odd and I am sceptical as to whether or not you have been furnished with all the details.
For one, the arresting officer would have to explain the details of the arrest to the Custody Sergeant at the Station.
Unless there have been thefts or similar in the area and your son fitted the description of person/persons involved I doubt that A. He would have been arrested for 'acting suspiciously' and B. that the Custody Sergeant would authorise his detention on those grounds alone, unless there are further enquiries pending, which by the sounds of things there are not.