Corporate Espionage?

Please excuse the dramatic title, but I have a strange situation with a potential customer unfolding. Our business is primarily b2b and we do business with prominent companies in our industry, supplying them components for their products. Recently we had a company that is out of our country reach out for a quote for a large volume of product. The relationship seems to have started out well with them hearing of us through our great reputation. We currently do business internationally and we have never had this request before. As we communicated with them they have started insisting that we send them photographs of our manufacturing facility ahead of purchasing any product and have said that they may also require a facility tour. Our factory is rather small and we have several proprietary operations that would show how exactly we make our products. Because of this we do not usually provide photographs or factory tours to anyone in order to keep our methodology private. Is it common place in manufacturing for customers to request factory pictures or detailed tours prior to even receiving a sample of our product? Or does this sound suspicious?