Which investigative technique involves observing a subject's activities?

Surveillance is an investigative technique that involves monitoring a subject’s activities to gather evidence about their behavior, movements, and interactions. It can include physical observation, the use of cameras, or other methods that allow investigators to collect real-time data on the subject’s actions without their knowledge. This method is particularly useful in fraud investigations where understanding a subject’s behavior in a natural setting can reveal patterns, inconsistencies, or confirm allegations of wrongdoing.

The other techniques mentioned serve different purposes. Interviewing witnesses focuses on gathering verbal accounts or testimony from individuals who may have relevant information about the subject or the case, while financial analysis examines financial records for anomalies or unusual transactions. Data mining involves analyzing large sets of data to identify trends and patterns, often using software tools, which is different from directly observing a subject's actions. Each of these methods has its unique role in an investigation but none specifically involves direct observation of a subject's activities as surveillance does.