Here's the closest recipe I've found and it has no tomato sauce in it:
1 cup salad dressing or mayonnaise1/3 cup relish1/3 cup French dressing (the orange not the red)1 tbsp sugar1/4 tsp black pepper1/4 tsp dry minced onion
Mix everything together. Will keep for a couple of months in the refrigerator.

I think it's mostly just tomato sauce and mayonnaise mixed together, though it might have something else, as it's supposed to be a "secret recipe" (http://en.wikipedia.org/wiki/Special_sauce#Special_sauce).
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MacDonalds' Big Mac. Remember the commercial?
"Two all beef patties,
special sauce
, lettuce, cheese, pickles, onions, on a sesame seed bun."
Maybe that commercial was just aired in Canada.

My guess would be Miracle Whip. It's pretty nasty!