Gao Guang spotted the guerilla camp.
The so-called camp was just a scattering of wooden huts under the trees; such a camp could only be discovered by ground searches. Aerial reconnaissance was impossible because the tree canopy above the camp was too dense.
Very little sunlight reached the camp, giving it a somber atmosphere. Moreover, the vegetation that shielded the ground was mostly gone, so one could see quite a distance within the camp.
Of course, "quite a distance" is relative; Gao Guang couldn't see the other end of the camp. He could only see the wooden huts stretching into the distance before being obscured by the forest.
The huts in the camp varied in size, but it was fairly easy to distinguish which hut served as some kind of headquarters. Those huts were not only larger, but they also had more people coming in and out, with more guards nearby.