In the case of Miranda v. Arizona, the Supreme Court ruled that police officers have to inform people of their constitutional rights when they are about to be questioned. The police must inform the people to be questioned of the fact that they have the right to remain silent and to avoid incriminating themselves, which is protected by the Fifth Amendment.
There are many specific situations, however, in which the meaning of this decision is not clear. Therefore, there have been many cases argued over the specific meaning of Miranda in given situations. One of these was Illinois v. Perkins. In that case, a prison inmate, Perkins, confessed to an undercover police officer that he had committed a murder. He thought the officer was actually a fellow inmate. The question was whether Perkins needed to be informed of his rights by the undercover officer in order for the confession to be valid.
The Supreme Court held that such an officer did not have to inform the suspect of his rights. They reasoned that this was not a coercive conversation. The situation was not one in which Perkins could have felt that he was being forced to incriminate himself. Thus, the Court clarified Miranda to specify that it does not apply in cases where the suspect does not know he is being questioned by police and therefore cannot feel compelled to confess.