Vertical BendyRuler - how does the copter know when to sink again

I’m attempting to use vertical BendyRuler with four sonars around the vehicle, but there is one thing I don’t understand:

After the copter has increased altitude to avoid an obstacle, how does it know when the obstacle was passed and it is safe to resume it’s normal height again? Obviously, a downward facing sensor might help, but what if there is none? The person on the object avoidance page performing the vertical test only has a front facing 1-D lidar either.