Some applications aren't ended completely when your mobile phone returns to standby mode. They keep running in the background unless you end them from the list of active applications.
End active applications
Tap the Home key twice.
At the bottom of the display a list of active applications is displayed.
Tap and hold any menu item until the menu items start to jiggle.
Tap - in the top left corner of the required menu item to end the application.
Tap the Home key to return to standby mode.
*Please refer to the Apple website for more information or further support.*