Its not food and theres no way a pine cone could have gotten in there but one of my rainbows were kind of swimming back and forth over it for about 3 hours today everytime a cory goes over there the rainbow runs over to it weird.
From the way you describe the rainbow's actions, it almost sounds like eggs, but if my memory serves correctly, they lay eggs in plants and show no brood care. It does look a little like a chunk of a pine cone though. I have found the weirdest things in my tanks, the most recent being a green bean. Maybe one of my sisters wanted to feed the fish?