Get yourself one of those USB pinhole inspection cameras (with LED light) and stick it in that limber hole, perhaps with a bit of wire coathanger attached so you can steer it. Then you can see the uncoated fibreglass inside the hollow stringer. This will tell the trained eye what's going on in there, whether the stringer is detached, delaminating (white areas) or if it's just cosmetic. Take lots of photos.
A small impact could cause this damage and the rest of the stringer and keel attachment would still be strong enough to keep the keel from moving, but it should then be repaired by grinding off the brown flowcoat and laying up more glass over the area. During grinding, the extent of any delamination will become visible. It's a messy job though and depending on where it is, you may need to remove some/lots of furniture to get there.
