If you espy something, you see or notice it. Here, from a window, did Guinevere espy a knight standing in a woodman's cart. to suddenly see someone or something = spy (espier; SPY)
To catch sight of; to perceive with the eyes; to discover, as a distant object partly concealed, or not obvious to notice; to see at a glance; to discern unexpectedly; to spy; as, to espy land; to espy a man in a crowd