The app should know when the Leaderboards / Achievements overlay is closed in certain circumstances. For example a player finish a level, the leaderboards overlay appears, user close it and the next level starts automatically. Is there ay way to get notified that the overlay was closed? Either by tapping the cross at top-right of the overlay, or pressing hardware back button? Have not found any listener / callback for that.
I assume by "Leaderboards / Achievements overlay" you mean the GameCircle UI popup? Unfortunately there is no API provided to allow your app to be notified when the popup is closed. Your app should be able to control the game flow independent on the popup closing event. However, I'm going to forward this feedback/suggestion to the GameCircle team for consideration.