Time to Play! #16, and the answer is…
Thank you for all your pertinent comments!
The dog was a spayed female (spay performed a long time ago), and therefore it is unlikely related to a post-surgical complication. The lesion was located caudal to the right kidney and as you will see on the answer-video, a CT performed as part of the follow-up demonstrated that this is a fatty structure (with a Hunsfield unit of -60). Several rechecks showed that it did not changed over time, supporting that it is incidental and likely a focal fat necrosis.
Another example is illustrated on the video “Idiopathic Fat Necrosis” available through the 2d edition of the Atlas of Small Animal Ultrasonography, or the PREMIUM videos membership.