2009 RX8 Check engine Light

I have a 2009 RX8 with 26,000 miles. I bought the car a year ago for my son so it is driven daily to school and his friends houses but not very far. I have had no issues with it and the previous owner records just show regular maintenance completed. Last night after starting the car to go put gas in it the check engine light came on. His father had just added oil and coolant to the car. Neither were low to the point of having issues but since he was visiting he checked them. He was thinking that it was possible that my son had let the gas run low causing it to come on per the manual. (He does have a habit of waiting until the gas light is on or about to come on) I filled the car up and when I got home he disconnected the battery. The light is not currently on but when I went to Autozone they said it was a mistake to reset the light and now they can't tell me why it came on. Would the Mazda dealer be able to help me or do I have to just wait for it to come back on. I would prefer he not drive it if there is a chance he could be doing damage to the engine. I am not sure if there is a issue how long it takes to come back on. Car seems to be running fine. Thank you for any help in Advance!!