Yes I found them in the gizzard also once found 3 marbles in ones gizzard no idea how as I dont have marbles lolI would find rocks in the gizzard (in the middle of the chicken.)
I would find grass and other green stuff in the crop (sack at the base of the chicken's neck, where the food goes before it gets ground up.)
I have never bothered to withhold feed before butchering, so the crop has typically given me a good idea of what they ate in their last few hours.