They stayed low, weaving through the streets rather than flying above them where they could be more easily spotted by the griffons. There were plenty of other ponies out in the streets, having heard the explosions, with several more having gone off since their flight began, and it also meant they were able to hear the calls of the griffons. It was easy to tell they were frightened, and they had good reason to be, especially if they could see what was happening in the park.