C'mon y'all, take a deeeep breath -- it is surely NOT a kidney, I have a generally poor opinion of vets in general but they do abdominal surgeries enough and are nonstupid *enough* that they can RECOGNIZE a kidney, normal or abnormal, when they see one. (e/t/a - and a normal cat kidney is only about the size of the last joint of your pinkie finger, even a *massively* pathological polycystic kidney would rarely be as big as that object is, speaking as someone who *had* a cat with massively polycystic huge kidneys. and if it *was* a giant polycystic or tumerous kidney, which it CANNOT HAVE BEEN in my opinion, it was not doing the cat any good in that state anyhow, end of subject)
It is not uncommon at all to have tumors that are encapsulated like that, a sort of organ-like distinct mass that is separate from the rest of the body tissues.
I have no idea whether it's likely to be cancerous and I don't think it's possible for anyone on an internet forum to have even a half-intelligent opinion on the subject, that is why vets send things out for biopsies
But certainly there ARE a number of noncancerous, or cancerous but non-metastatic and fairly curable, tumors that cats can get, and hopefully that is what this will turn out to be.
I really wonder whether that may have been related to her spraying, though. Not only is it good that you found out about this and had it removed (however unexpectedly it was), it is also *possible* it may help with the problem, and surely must make her feel better at least.
Best of luck, will keep fingers crossed for the biopsy to come back with good results,
Pat