Hex saw the female spy crouching behind a short wall behind the Soaking Twig. The tall growth on the main branch was akin to a small bush, providing plenty of cover for people needing to hide. Or to spy.
During the tour he received from Sterling, Hex had taken note of this place. At that time he had done so just in case he would ever need such a place, not knowing it he would be using it this very night.
The plenty of places to take cover was the main reason he had chosen this spot to meet her. It provided enough shadow to hide any inconsistency his appearance might show. The partial transformation was relatively new to him. He preferred to take precaution in case he missed a spot.
But it wasn't only for himself that he had chosen this spot. The other spies needed something too, didn't they?