- If the node is a leaf, just scan over the triangles and find the closest one. Return the index of the triangle and the distance to it.
- If the current node is not a leaf, figure out which side of the split plane that contains the query point, and recurse down that side. You will get the index and distance to the closest triangle in that subtree. Then, check if the distance to the nearest triangle is closer to the query point than the distance between the query point and the split plane. If it is closer, there is no need to recurse down the other side of the KD tree and you can just return. Otherwise, you will need to recurse down the other way too, and return whichever result is closer.
Keep in mind that the closest point on your mesh could be an edge or a point, and in that case you will have a tie ... this may or may not be a problem depending on what you want to do with the triangle index you get.