There can be a number of factors that lead to a small hole in an incision. 1. When dissolveable sutures are used, you can end up with a small stitch abscess that can drain a small amount and can cause an opening.2. You can have a small inclusion cyst. This occurs when cells from the outermost layer of the skin get trapped inside an incision. Because those cells keep dividing and turning over, they produce a white substance that can often be expressed out of the hole or can come out spontaneously.3. You can develop a fluid collection that works its way out to the surface of the skin. This is most often a seroma, but can also be a hematoma or an abscess. In this case, a tunnel from the outside down to the fluid forms and is called a fistula.4. Sebaceous cysts form from sweat glands and can occur in an incision if a sweat gland was cut (less likely in this location)5. Pilar cysts can form from hair follicles. Similar to an ingrown hair, a hair follicle can be trapped in the incision. This might account for pulling a hair out of the hole.6. You can develop a depression that looks like a hole. The top layer of skin is called epidermis. Sometimes if the edges of an incision aren't lined up perfectly, there can be a step or irregularity in the incision. Once the scar forms new epidermis, the contour will stay that way. Similar to a pierced ear, where the tract stays open because there is new epidermis lining the hole, you can have a depression in the incision.
In any event, it is worth seeing your surgeon to have the wound assessed and treated. This is most-likely a minor issue and can likely be resolved easily. Good luck!
Local infection arounf belly button
This seem to be a small granuloma (local infection around suture or hair follicle) or a fistula. Evaluation and treatment by your surgeon would be your best option. Good luck.