George and Lynne are in a tent. Lynne asks George to remember that it was pitch black when they arrived last night. George does remember and adds that he didn't think it was the camp site but Lynne wasn't convinced. Lynne exits the tent to find that they are in the middle of a football stadium. She admits she was wrong.
George and Lynne have gone camping and arrived to what they believe to be the camp site in total darkness. This means that George didn't even have his lights on when they drove, although he may have had them on a low beam when arriving at the stadium, which Lynne thought was a camp site.
Following that error, he must have then found a way to drive onto the pitch. Some stadiums have a tunnel which leads from where the coaches would arrive, past the dressing room and onto the pitch, so that is understandable. George did not want to wake the other campers he believed were there and sleeping so, at this point, he completely turned his lights off and drove slowly towards a clearing that he had spied earlier.
It is now at the earliest midday so George and Lynne have slept through the warm up and all the crowd arriving. The question is not how they did not know they had camped in a football stadium but why the groundsman didn't wake them up and remove them before the game started. If Lynne had not woken, were the teams just going to play round them? There must be some old mediaeval law that states that if a man pitches his tent within the city walls, then he is not allowed to be moved.