Does the Prophet’s — may Allah’s peace and blessings be upon him — saying: "By Allah, neither your devotion nor your bowing is hidden from me; indeed, I see you from behind my back," indicate that he knew the Unseen?

Allah alone knows the unseen, and He may inform whomever He wills of His messengers about whatever He wills of His unseen knowledge. This is what happened to the Prophet, may Allah bless him and grant him peace. His ability to see what was behind his back during prayer is a miracle with which Allah specifically endowed him. It is not considered knowledge of the unseen, but rather a vision of something existing and observable, in which the ordinary course of events was transcended.