I have rock checker scouts. They visit every colonizable rock with a res 1 active sensor. The range is pitiful in deep space, but since they get within 10,000 km, they can detect anything detectable with an active sensor quite economically.
Much cheaper to have them visit anything you are considering landing on before sending colonists and infrastructure there. A fraction of the cost of a survey ship, and they don't need anywhere near at much time to scan each rock.