If you ate (or prepared) a sandwich at home, you will soon have an opportunity to improve your situation; if you ate the sandwich in a restaurant or at a lunch counter, it is a warning against confiding in casual acquaintances.
Toasted sandwiches or sandwiches eaten outdoors on a picnic suggest that you are contemplating (or involved in) a love affair which could cause you serious embarrassment; in either case, think again. [1]
To see or eat a fish sandwich denotes an inner struggle between faith and reason. [3]