It's fine to detain a suspected thief yourself to get a confession.

Even if someone is genuinely suspected of theft or wrongdoing, privately confining them to force a confession or recover property is itself a punishable offence; such matters must go through the police and courts.

Indian Penal Code, 1860

Section 348

repealed

Wrongful confinement to extort confession, or compel restoration of property

Whoever wrongfully confines any person for the purpose of extorting from the person confined or any person interested in the person confined any confession or any information which may lead to the detection of an offence or misconduct, or for the purpose of constraining the person confined or any person interested in the person confined to restore or to cause the restoration of any property or valuable security or to satisfy any claim or demand, or to give information which may lead to the restoration of any property or valuable security, shall be punished with imprisonment of either description for a term which may extend to three years, and shall also be liable to fine.§

Why this exists

This section addresses a specific abuse pattern: people taking the law into their own hands by illegally detaining someone to force a confession or recover stolen goods, rather than going through proper police or legal channels. It protects individuals from private coercion, even when the underlying grievance, like theft, may be genuine.

Common misconceptions

Myth: It's fine to detain a suspected thief yourself to get a confession.
Fact: Even if someone is genuinely suspected of theft or wrongdoing, privately confining them to force a confession or recover property is itself a punishable offence; such matters must go through the police and courts.