I'm assuming it's a glacial lake by your location.
Clay bars are fairly common in these lakes, from what I have observed. A seam of clay that intersects the dropoff, or is exposed near the lake bottom, will have the softer sediments eroded away around it by currents, and possibly also the action of fish.
Going back and scanning this target on both sides, and again at right angles to the first 2 scans, will give you a much better idea of the object's shape.
If water clarity allows, I would drop a camera. You can always use the age-old method of bouncing or dragging your anchor on it to determine the composition!
