Which method of surveillance involves constant observation of a subject?

Close surveillance is the method that entails constant observation of a subject. This approach is typically employed when detailed and immediate information is necessary about a person's activities, movements, and interactions. In law enforcement practices, close surveillance is used in situations where the subject may be engaging in criminal activities, or when it is crucial to gather real-time intelligence.

The nature of close surveillance allows investigators to maintain a continuous watch and respond quickly to any developments related to the subject's behavior. This is particularly valuable for ensuring evidence is collected accurately and for providing insights into the subject's patterns and potential criminal involvement.

Other surveillance methods like loose surveillance involve less scrutiny and are more about general observation without the intent of constant tracking. Combined surveillance is a strategy that incorporates more than one method of watching over a subject, and infrequent surveillance provides only occasional observations, making them less effective for situations that require immediate and ongoing attention.