W166 loud beeping and red seat belt indicator - possibly faulty seat belt sensor?

[RockDriver]

Today I bought the 2013 W166 ML350 Bluetec, was driving around with it and one time as I was just pulling out of a fuel station this rather loud beeping started with red seat belt indicator on. The beeping was different from the usual "ding" sound when you start driving with the seat belt off, although the car is completely new to me, so I could be wrong.
I stopped the car, turned the engine off, buckled the seat belt several times and the beeping was gone. Later the same situation repeated again once more. So I don't know what actually happened - is it possible seat belt sensor is faulty?

 

[davidsl500]

My R172 gets more "insistant" after a short time if my better half unclips her belt early in the supermarket car park. Did you have anything heavy on the passenger seat that the car thought was a person which you moved?

 

[RockDriver]

My R172 gets more "insistant" after a short time if my better half unclips her belt early in the supermarket car park. Did you have anything heavy on the passenger seat that the car thought was a person which you moved?

Now that you mentioned it - I had my backpack. Not particularly heavy, but maybe heavy enough. I need to try it with a real person. But the beeping was mad loud - is that what it normally sounds like? I had over a dozen cars (3 of them Mercs) before this one and they all did the classic "ding" sound which is very different.

 

[davidsl500]

It's some time ago but from what I recall it starts normally with the the light chiming sound but then after around 30 seconds it gets much louder, maybe with a different tone. My better half has certainly not taken the belt off early since!