Tracker will show up occasionally once you place his special Cauldron in your village. With his cauldron your smurfs will get a special soup that gives bonus XP every time you earn XP in the game.
(Please note that in order for him to show up you must be connected to the INTERNET at all times. If Tracker disappears after closing & returning to your game, minimize your app and wait a minute and then re-open it. If that still does not work, exit to the Smurfs’ Village main menu and re-enter the game again. Doing both of these will force a re-sync with the Beeline server.)
Tapping on Tracker or his Cauldron will display the current task list in his scavenger hunt XP event. By completing the tasks in his scavenger hunt, you will make the soup stronger. (Please note that with each scavenger list completed, your XP multiplier will increase.)
You can keep completing the tasks on the scavenger hunt list until the soup & time runs out. Once the soup is gone, tap on the cauldron and receive your massive bonus XP!
You can find more details about Tracker smuf here.