Afterward I commented to a friend/colleague who sings in the choir on how special the Benedictus was. His face lighted up as he agreed with me: apparently it is a strict canon for three voices.
A conversation with a mutual friend brought up the difference between listening for structure and just listening. I maintain that it was the special structure that caught and held my attention right from the start. That's what I call a successful structure, for that is its purpose. Whereas, to me, listening for structure is to risk missing the point.