What is this really tasty part of a chicken found in a cavity in the hip?
Those are the chicken oysters -- muscle meat, not organ meat. I'm glad you've learnt to enjoy them by intuition, as they are indeed a prized portion of the chicken.

Wikipedia tells me the French call this portion sot-l'y-laisse: "(only) a fool leaves it there", because it is little known, easily missed, and much prized.