If a security guard comes upon a crime scene, which action should they take?

When a security guard encounters a crime scene, the most critical immediate action is to notify the police and prevent anyone from entering the scene. Ensuring that the area remains secure is vital for preserving evidence and maintaining the integrity of the investigation. By securing the scene, the guard can help prevent contamination or tampering of evidence, which could impede law enforcement's ability to conduct a thorough investigation.

While the other options might seem beneficial in theory, they are not appropriate actions for a security guard without specific training or authority. Placing physical evidence in secure plastic bags could inadvertently damage or alter that evidence, especially if the guard is not trained appropriately in evidence handling. Similarly, marking evidence with chalk is typically the responsibility of law enforcement or trained forensic personnel to avoid introducing biases or altering the physical evidence.

Consequently, the most responsible and effective action for a security guard at a crime scene is to ensure that the police are notified immediately and that the area is secured, allowing trained professionals to handle the situation from there.