What must every agency do upon taking a person into custody or identifying a missing person?

The correct action that every agency must take upon taking a person into custody or identifying a missing person is to place a locate on the corresponding record. This step is crucial as it updates the national databases, such as NCIC, to reflect the current status of the individual. By placing a locate on the record, the agency ensures that other law enforcement agencies have access to the latest information regarding the person's custody status or missing person status. This facilitates smooth communication and cooperation among agencies and prevents multiple entries or outdated information from complicating investigations or searches.

The other options, while potentially relevant in certain circumstances, do not represent the primary required action in this context. For instance, initiating a national alert may occur in specific scenarios, but it is not a universal requirement for every agency at the moment of custody or identification. Similarly, notifying local law enforcement could be part of procedural protocol but is not a primary action mandated by the data management requirements of NCIC/TCIC protocols. Conducting a background check might also be performed, yet it does not directly address the immediate need to update the individual’s status in the system.