Why should the narrator fear being discovered?

If discovered, Peter will likely be executed for his part in tearing down the wall, as well as for attacking Sir Phillip. He might also be killed on the spot..... one shot came very close.

I felt the bullet whizz through my hair—which was standing on end, I expect, for I'd never been fired at before. I'm not exaggerating. The cap was blown clean from my head, and fell somewhere among the rocks, where I'd neither time nor inclination to stay and look for it.