Who knows the circs? Occasionally in the popice an officer might get sent on a course or suchlike in another part of the country, and quite often they'd be hired a car.
Usually it would be a Corsa or such, but i once threw my teddies and pointed out that being the size I am I can barely sit beyond the wheel of a Corsa, could not close thendrivers door while sested, much less operate one (I knew this as my eldest had a Corsa herself at the time) so they would have to get me something a little larger
I was expecting a basic Focus, and was quite surprised to find a Galaxy Titanium with all the trimmings. I suspect it was the cheapest thing bigger than a Corsa they could get.
So it could have been for similar reasons that this woman had the car, although a Merc seems a bit frivolous when a similar sized Ford or Poxhaul would provide equally adequate transport and be much cheaper.