That is the KG reminder warning telling you to not leave your
keys in the car. All MB's with Keyless-Go display that with a door
open. It is not defeatable, The only way to turn it off is to tell
the Cluster that the vehicle does not have KG. The draw back of that is, KG will become inoperative.
Maybe someday down the road we will have the programming ability to turn just that beeper off. Unfortunately not yet.