It's a reminder that you left the remote in the car. It also simultaneously displays a message in the Driver Information Center (little blue screen). If you normally do leave the remote in the car (e.g. in garage), you can turn this feature off via the Config Menu.
Three beeps is it thinks you left the remote in the vehicle. One beep means remote leaving vehicle proximity and you have left doors unlocked and it may also lock them at that time for you. both can be turned on or off through the configure menu as sated above.