I did a few months for a supermarket call center answering queries about products, so had to look up the product details, there is no way checkout staff could possibly know in depth details on hundreds of products.
It’s better to say you don’t know, possibly loosing a sale than mislead the customer
It was years ago now, but a niece visiting from the US used to have a severe egg allergy. (She’s grown out of it now,).
My sister asked at Asda customer services for a list of all the products containing egg, and TBH I was impressed when they produced one, no problem.
There is egg in so many things you might not suspect, including some sausages.