It's possible they don't have salad dressing in the French Dressing, Thousand Islands mould. After all Spaniards generally dress their salads with oil, vinegar and salt and don't feel the need to add strawberry or mustard flavours. Mercadona is now the biggest retail outlet in Spain but one of the secrets of its success is to sell its own brand and to keep the choice fairly limited. If Mr Mercadona, Juan Roig, doesn't think they'll shift enough salad dressing off the shelves they won't sell it.
I've lived here a while now. I've been to supermarkets hundreds of times and I still don't know where to find those "every now and again" articles even in the shops I use regularly. It doesn't help that there is no master plan for Mercadonas (or Carrefours, Eroskis, Más y Máses, Días etc.) I was brought up on supermarkets with similar layouts. Tesco in Birkenhead was arranged much like the Tesco in Sevenoaks as I remember. The Mercadona in Plaza de España is completely different to the one in Calle San Juan. My advanced age also has some bearing on my shopping abilities.
It was, as I shuffled from one end of the store to the other time and time again, that I realised there is a more fundamental difference.
I have an English sensibility about the layout of a supermarket. Booze is drink so I expect it to be near pop and juice. Tea is a drink like coffee and hot chocolate so they should be close together too. Rice and pasta go together in my mind as dried foods so the dried pulses will be there as well. Following the same logic, dry goods, close by will be things like flour which means that other cooking staples - baking powder, desiccated coconut - will be near - maybe the microwave popcorn too. Spanish people think differently and so the shelves are stocked in a manner that manifests the difference.
Never mind. In the search for salad dressing I had a good poke around and came across frozen pimientos del piquillo stuffed with bacalao. They'll be a bit of a treat one evening.