There are several situations where problem foods may not be hidden, but there is a risk of exposure. A good example is at the salad bar: even though the chick peas may not be near the croutons you like, we never know if someone has mixed up utensils or simply used the same for both. The jelly jar at a friend’s house may be far from the peanut butter, but the same knife has been used for both previously.
This can also happen at the ice cream parlor, where rinsing the scoop between customers may not be done properly and traces of an allergen can be mixed into another person’s serving. You should say, “I am allergic to XXX; please rinse the scoop in running warm water before scooping out my serving.”