Howdy Samuel,
There is a few options here.
Just above the posterior iliac crest you can get a triangle of Petit (inferior lumbar space) hernia that can look like you describe.
A little bit higher you can get a space of Grynfelt (superior lumbar space) hernia.
I have attached a video file of a space of Grynfelt hernia where I thought the kidney was going to fall out!
On the other hand, you could be looking at normal adipose.
Do you have images or clips for me to review.
Se you in London soon!!!
Steve