Final answer:
The common allergen present in the mentioned food items is soy, also referred to as soya, due to potential cross-contamination in the facility where they are processed.
Step-by-step explanation:
The common allergy present in salsas, death relish, and guacamole, according to the information provided, is soy. The allergens list mentions that items are made in a facility that also processes products with gluten, milk, soya, celery, sulphites, and their derivatives. Since 'soya' is another term for soy, it is the allergen that is most likely to be found across all the products mentioned due to cross-contamination even if it is not an ingredient in the salsas, death relish, and guacamole themselves.