What is this really tasty part of a chicken found in a cavity in the hip? About once per month, my mother prepares chicken with rice and a currysauce with pineapple. I don't know exactly what the method of preparation is of the chicken because it's prepared by one of those traveling grilling trucks.

However, what I do know is that there is a small (like 2 cm wide and half a cm deep) cavity in one of the hips of the chicken that's filled with really tasty meat. The cavity is open on the  inside and I think it's the best part of the chicken. It appears to be some kind of organ meat, but I'm not sure what exactly it is. I also don't have a picture of the meat, sorry.

What could this meat be?
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.