From outside, the entrance seemed to just be a shed, so he knew that right behind the door was a small landing area just before a continuous staircase underground.
The door of the prison swung open, and Eagle dragged the guards back to the prison after taking their keys. He was careful to peek and check if there were people behind the door. Fortunately, there was none.
It was likely because of the war, and some people assigned here were also sent out.
However, it was safe to assume there'd be a few at the bottom of the staircase, especially when the two guards were not just trying to get their keys when they thought backup was coming—they were trying to press a button too.
It was probably an alarm or something, and fortunately he used several skills to imbalance them enough so they couldn't get near it. Anyway, Eagle kept as quiet as possible.
