Under what circumstance can a security guard legally detain a suspect?

A security guard can legally detain a suspect if they have directly witnessed a crime occurring. This is based on the legal principle of "citizen's arrest," which allows individuals to detain someone if they have reasonable grounds to believe that a crime is being committed in their presence. This principle ensures that the security guard has firsthand knowledge of the criminal activity, rather than acting on assumptions or third-party information.

The other options do not meet the legal criteria for detention. For instance, a suspect being unarmed does not provide justification for detention; in fact, the presence or absence of a weapon does not determine a guard's authority to act. Similarly, a mere complaint being filed is not enough, as it does not necessarily imply that a crime is currently occurring or that the security guard has witnessed it firsthand. Detaining someone based solely on a hunch lacks the necessary legal backing and could lead to unlawful detention, which can result in legal repercussions for the guard. Therefore, witnessing a crime is a fundamental requirement for a lawful detention.