First of all police don't remind the suspect, it only arrests the suspect of any assumed crime, and then produces before the court who either releases him, keep under police custody or sends him to jail if he is suspected of any non bailable offence etc etc. FIR is not must only suspicion is enough to arrest anybody. Rest is with the court.
It is often the case when the police arrest a person in suspicion of a crime they are unable to complete the investigation in 24 hours. At this juncture when they require the accused or suspect to be kept away from society at large for the protection of society, of the accused or for the purpose of ensuring his availability for investigation, they may produce him before a magistrate, who may allow for the suspect to be held in the custody of the police or the judiciary.