i think it was never put on, to free up cash flow. Theirs. Catalytic converters are expensive.
To clarify: Catalytic converters get old, stop working, need replacement, just like belts, lubricants, etc. You said the car was "used", so I assumed, perhaps wrongly, that it was old. In many cases, catalytic converters need replacement in under ten years, and they are EXPENSIVE. Maybe not for Mercedes. I don't know.
If I had an old bomb of a Mercedes, or even a nice old Mercedes, and it needed a new CC, I might think that I didn't need to put out the several hundred dollars for a new CC.
I don't believe it was stolen. I can't imagine a market for used, or partially used CCs. Although I'm also able to imagine there might be such a thing with the level of intellect out there.
Of course, they could have left the old CC on the car and just had it not being effective. Then it would have looked fine.