I jogged him out. He was sound. I carved at it with a hoof pick. It collapsed in a bit more. All the while, Speedy stood there half asleep, occasionally checking for treats as I rested his hoof on my thigh. Nothing I did elicited any kind of pin response, and Speedy's not particularly stoic.
Our best diagnosis was that it might have been an old abscess that never surfaced. That didn't ring true though as he hadn't been unsound. And with Speedy, abscesses have hurt enough to make him really lame. Our other thought was that he whacked it some time ago, forming a bruise. It could also be something completely different. Whatever it was, we surmised that during our ride the day before, his hoof must have flexed enough to weaken the damaged part of the hoof which then flaked away.
Needless to say, I haven't called the vet yet. It seems that whatever it was, it resolved itself long ago, and today I am just looking at the remnants. My farrier is due in early March. I'll let him tell me what he thinks it was.