Final answer:
Mousses contain allergens such as egg and mustard, and they may also have traces of gluten, milk, soya, celery, sulphites due to cross-contamination in facilities processing these substances.
Step-by-step explanation:
The question asks which allergens are found in all mousses. Based on the allergen information provided, all mousses contain egg and mustard as allergens. Additionally, they are made in a facility that processes products with gluten, milk, soya, celery, sulphites, and their derivatives. While these may not be in every mousse, cross-contamination could occur, and individuals with severe allergies may need to be cautious.