This reward is received every time you light up an enemy vessel that has been invisible for at least 30 seconds.
It doesn't take a whole lot of imagination to see how a destroyer in particular could earn a tremendous amount of experience and credits without ever firing a shot. If a destroyer could sneak over to the far side of the map and continually spot the enemy team, the rewards should be really high, right? Well, kinda...
Team composition can largely dictate how much experience will be earned in a given match from scouting. Aircraft carriers can dominate the spotting and detection gains, diluting the amount of rewards earned by surface vessels. Radar equipped ships can keep destroyers at arm's length and limit the amount of movement around the map. A battleship-heavy game will have less spotting opportunities as their gun bloom will make them visible to almost everyone on the map, barring island cover.
Scouting rewards are divided among all of the ships providing vision.