There are 13 districts. In each district you'll find 1 secret agent, marked by a silhouette of a man wearing a hat. You'll be shown a picture of the secret agent you must defeat. If you defeat them before they see you, it's easy. If they see you first, though, they'll run and you have to chase them down. There's a high potential for collateral damage to civilians so I would definitely recommend doing this on your Evil playthrough.