I think it's the souls of brewers past.
Edit to add more than just humor...
Of course I'm no expert, but having read Palmer's book and listened to him discuss it. I think you're both right. The foam just before boil is hot break, and so is the stuff swirling around once that foam falls. My understanding is that at boiling the pH drops due to calcium precipitation, plus you have heat and motion. This combines to cause proteins and tannins to collide and combine into big masses. Then at chilling they sink to the bottom along with all the other particulates, like hops and whatnot.
Whirlpooling sets the contents of your pot into a circular motion, the outside is moving faster than the center. When you stop whirlpooling (as long as you don't disturb it) the stuff tends to settle near the center where it was moving slowly. In my kettle I whirlpool through the whole boil and chill, then cut the whirlpool. I then take a hydrometer sample. Once the break in my sample has settled enough to get a reading, I gently run off to the fermentor. That's generally about 15 minutes.
Or its the souls of dead brewers.