"Locking down temporarily pending investigation. Please contact the iCloud account the Mac is linked to."And it was asking for a PIN code.
Many calls to Apple Technical Support and one visit to the Apple Store later it turned out that this Mac had been placed under an iCloud lock. This is a feature that is normally used by the owner of a Mac to lock it down when it is stolen. Except that I was the owner of this Mac. I had bought it from a private party three or four years earlier, I cannot recall exactly. (But see the postscript below.)
Now, Apple has the ability to remove an iCloud lock, but they refused to do it in my case because I could not prove that I owned the machine. They wanted to see an "original receipt", which of course I don't have. I suppose it is possible that the machine I bought was stolen, except that I have to wonder why the rightful owner waited years before locking it. If the machine is stolen, I would like to see it returned to its rightful owner. But the cryptic message on the lock screen gives me no way to contact the person who had initiated the lock. Apple knows who that person is, but they won't tell me, which is understandable. But they also won't contact this person on my behalf, which is less understandable.
I wrote a letter to Tim Cook to no avail. He almost certainly never saw it. I got a call from a lackey who politely but firmly told me that Apple was not going to change their policy. They will not unlock the machine without a receipt, and they will not contact the person who placed the lock.
So I am hosed. I have a locked machine, no way to unlock it, and no way to contact whoever placed the lock. I can't even safely dispose of this machine because there's personal data on the internal drive that I now cannot erase.
FWIW, the machine is a 2010 11-inch MacBook Air, serial number C02DM1GNDDR0. If you are the person who locked this machine, please get in touch.
Postscript: When I found out that my machine was under an iCloud lock and Apple wanted proof that I was the owner before they would remove it, I went back through my records and found the correspondence I had with the person I remembered buying it from. I also went through our basement and found the box that it had come in. I figured if the machine was stolen, the thief would probably not have taken the box, so the fact that I had it would be pretty convincing evidence that it wasn't stolen.
Unfortunately, when I checked the serial number on the box against the locked machine, it turned out that they didn't match (even though the model was an exact match). What I think happened is that I had bought a second, identical machine at some point and then re-sold it (I have a vague recollection and some sketchy records of this second transaction). When I sold it I must have used the wrong box. The serial number on the box is C02DPD69DDQX. If you own this machine, please contact me. I have your box, and you may have mine.
Lessons learned: If you buy a used Mac from a private party, always take it to an Apple store to make sure that it is not bound to an iCloud account. If you don't do this, you don't really own the machine. It turns out that the iCloud lock is implemented in the UEFI secure boot ROM. Wiping the hard drive and doing a clean install of the OS is not enough to disable it. Make sure you get and keep the sellers contact information. Check their ID. And, of course, keep track of the box. (And check the serial numbers!)