Specking Spanish

My dad works at Theisens and he see a lot of Mexicans that come in and will only speak spanish when they know English. He said that one day they were sitting there talking in spanish and somebody understood what they were saying and they were making fun of someone. So dad went towards them and they all of a sudden they stopped talking and turned around. Then one day I was in there and seen a couple Mexicans. I went in the next aisle and tried to listen to what they were saying but they went way to fast. You could still here what they were trying to like, express, in their tone of voice. Then I could tell if they were confused trying to look for something or actually talking about someone. All they would speak was in spanish when they could easily spoke English. They only spoke English when they went up to the counter and had to, but thats probably also what we would do to. If we were in a spanish speaking country.