Depression/Groove on back of Octave Mandolin

[Petty1818]

I have a Trinity College Octave Mandolin that is in great shape. I took it to a luthier about a year ago and while inspecting it, he noticed a small depression on the back close to the middle of the body. You have to hold it up to a light to even see and even then it's difficult. I find that I only notice it when I run my hand over it. It's a small depression.

The luthier asked if I humidified it and when I told him I did, he just suggested that I not let it get too dry. He checked the inside and the rest of the guitar and said it looked solid. Since then, the guitar has remained the same. I have no issues with the action or top or anything.

Is this something that I should be worried about? I might take it in again to get checked on but is it possible that the wood just has a slight abnormality where there's a small depression?