Obvious in the sense that they had no clue what a celiac is and they've ordered a sandwich previously. You can't become celiac from one meal to the other. That's not how it works.
No, but you could be trying to work out if you are celiac or not. The symptoms are identical to those of IBS and lactose intolerance. Most people with stomach issues do not know the cause.
In order to work out if you are celiac you have to cut gluten out of your diet and see if your symptoms improve.
And if you find out you are celiac then it's obvious that you would then stop eating gluten.
It's ridiculous of a restaurant/cafe to kick someone out for asking for gluten free. They could just not serve gluten-free and that would be fine.
But if you serve gluten-free and a customer is going to pay for it then it's just crazy to decide that you can judge who can or cannot order it.