If the spring that holds it is place is rusted the ornament can come off really easy.
You can check it by removing the ornament, just one clip.

In the eighties they ware very popular as personal ornaments.
I lost a few but the imitation ornaments were cheap.

Once someone tried to steal it with a large screwdriver and poked it through the radiator.
I attached the new one with a switch and relay to a "Bosch Stark tone horn" (very loud).
According to my neighbours it worked to well, so I replaced it with a flat logo thing.
Fortunately wearing a Mercedes ornament went out of fashion.

Rob