It means that your scout has seen the player in action and so you can see his stats and skills. When you take the player off your scouting list the stats of that player disappear. Tbh I don´t really like this system in ML, it´s unrealistic as you should be able to see at least half the stats of famous players and the stats of the players you have scouted should´t just go away, the stats of the players should just stay still until you watch him again.