I was climbing with him that day, although not belaying at the time. Virtually the same thing happened to another of my friends and I last year on P2 of Duress/Under Duress. Solid looking block the size of a shoebox, didn't move when hit, didn't sound hollow, then flew off as soon as I weighted it. It hit my friend in the head, busted his helmet and knocked him out. It gashed his head pretty good, and almost surely would have killed him had he not been wearing his helmet.
I, like the original poster, always wear my helmet there when belaying, climbing, and walking along the wall. It's hard to believe there are people who don't.