Upon closer inspection, you note that the lid is shaped like a ducks head and there are pictures of bananas on the bottle. You point this out to your guide and he chuckles a bit, looks at the bottle, then tells you that it is actually banana-flavoured mayonnaise. You put it back on the shelf and:

decide that mustard really isn't all that important and move on to other shopping.

search a little longer for the ever-elusive mustard.