After much anatomical discussion, the mystery of this chicken part seems to be solved. You won’t find it in the baggie containing giblets.
In the cavity of some commercially dressed chickens there are two reddish-brown blobs that rest next to the lower spine. They ‘re leftover bits of the kidneys.
Karl S recommends checking for them and removing them. If left alone, they’ll add a slightly acrid note to the pan juices.