My spot lock keeps me, at best, in a 20ft radius of where I hit the anchor lock feature. I know it works off of satellite triangulation using a minimum of three satellites. GPS triangulation is accurate to 20 feet, so the boat has to move at least 20 feet before the triangulation changes enough to register on the unit , which will then kick the motor on and return to the original starting point.
I suggest anyone that is saying five feet throw out a marker buoy and check again.