Hi,
That translation key and that class are used in the file administrator/components/com_hikashop/classes/product.php, in the function checkVariant.
That's the function which is called to automatically fill the variant information with the information from the main product. It is used at many different places: the product page of course, but also when the cart is loaded, when the cart item is modified, when the contact form for a product with variants is used, when a contact email is sent, when the waitlist mechanism is used, etc.
It's done when the data is loaded from the database, not when the information is displayed in the HTML and that's why you can't find it in any view file but in a class file.