It is possible and very probable that they missed part of an ovary. It does happen a lot, especially if a vet is just starting out and isn't very good at surgery yet, though good vets know what they're doing. I think you can have a progesterone or similar blood test done possibly to see if the hormones are still in her system, indicating the remains of an ovary. They do testosterone tests when a male cat acts like that, because some vets won't take out a retained testicle and doing that test will show if one is present.