I bet it's got a mayonnaise base, and not Hellman's/BestFoods mayo either. Probably Kewpie mayonnaise. I see this a lot in Asian restaurants in the US.
As to citrus, Lemon or Grapefruit would be my first guess. Orange is used a lot but is usually sweeter than what you've described. Take a good look at the menu and see where citrus is called out in the descriptions. I suspect they use the same citrus in multiple applications as it makes little sense to source citrus just for the dressing. They might source solely for the dressing but it doesn't strike me as best restaurant practice.
Is the sour all citrus (citric acid) or is it more acetic, the acid in vinegars. They do taste different. The wildcard is that they might be using a citrus rice vinegar as well. I've liked those when I can find them in the asian grocers, but they're not that common.
Tan, well, does it taste asian? Think soy or sesame oil flavors. Those would both give some tanness, I lean toward sesame oil as a hunch than soy.