Does “rainbow bubbles” always mean leftover dish soap?”
I think rainbow bubbles can also be cause by the fats and oils in a dish. Think about the rainbows you see in oil slicks (bad comparison, I know). Even if it is dishsoap, it is probably in such small quantity that it would not make a difference to the taste of the soup. Dish soap has to be non-toxic, too, since it can so easily be left on items we eat off of. If the soup tastes ok, I wouldn’t worry about it. If the bubbles bother you, try skimming them off.
