Automatic body scanning? What is happening here? This has happened to me a few times  on Goenka retreats. 

When I have been practicing mindfulness of breath and body scanning for a while, near day 7 or so my attention will be moving quite quickly, rapidly, performing a scan of several points in the body every few seconds in a set pattern. By this point in the retreat my attention is flowing smoothly like water so it only takes a second or two for every cycle, which is very fast even if it can get quite tiring. 

At some point after this suddenly my attention gets hijacked by some autonomic process, and my attention will hit points in my body in a super complicated but precise pattern (I think it draws a mandala) that I can never hope to consciously replicate. It is ridiculously fast and precise, covering 50 or more  points in the body in under a second.

It happens only once, so a second later it is over, and now I note that my habitual set pattern I used to follow to scan the body is now in cruise control mode. I can actually sit back (metaphorically speaking) and let the body scan itself, or so it would seem. My mind will now go do other things, like contemplate dhamma or whatever, and when I return to attention of my body I would find it is still going strong, doing the auto pilot body scanning.

This will usually last a few days, even after the retreat. The funny thing is the attention is spatial. If I get up from the half lotus, the body scan still happens, but it is now hitting empty space near my hip where my thighs and legs used to be folded.

Often this means when I go to my room I won't lie down until I absolutely have to because it feels weird to have the body scan happen only to my upper body and not my knees and toes when I am lying flat. 

If I wake up from sleep the body scan is still happening.

After day 13/14 slowly this auto scan will grow less and less pronounced until I can't sense it anymore.

What is happening here?
This automatic body scanning that you mentioned is your ego playing games with you. During meditation, your Self or ego is becoming weaker and weaker. When the ego sees he is dying, ego will do everything to not die. You could hear sounds, vibrations, gain supernatural powers, travel to distant galaxies, predict the future, read minds, gain magical powers, fly through walls, visit astral planes, talk with devas, have out of body experiences, etc. 

Ego wants you to cling onto something. Ego wants you to crave. That's why he gives you weird experiences. If you show fear to such experiences, ego will give you experiences that you'll be afraid off. If you show attachment to such experiences, ego will give you experiences that you'll be attached to.

The important thing is not to get attached to these experiences and get beyond them. Don't worry, if the experience arises, just go with it, enjoy, relax, have fun and don't intellectualize. The most important thing: never, ever be afraid. If a bad experience arises, smile, don't worry, just go with it, enjoy, relax, have fun and don't intellectualize.

The closer you will be to liberation, the less such experiences will arise.