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?