I'm going through my passwords, updating everything.
While making some modifications to my keychain on my MacBook, I got to the following bizarre screen:
Now I did have an iMac — but I distinctly remember I called it piMac, And I installed a fresh MacOS before giving it to my brother to sell a couple of years back.
Luckily I remembered the password. But why on earth is Apple fishing out this ancient machine? It should have no knowledge of it.
So I Google and get iCloud: Manage your devices in Settings on iCloud.com
I follow the instructions, and in https://www.icloud.com/#settings I can see my MacBook and my iPhone. These are my only two Apple devices. This is correct.
How could the keychain possibly be referencing this ancient device?
I had recently checked the KeyChain item in the list visible in the screenshot — as I noticed it was unchecked. I figured it would be a good idea to have the keychain shared between the iPhone and MacBook so passwords could transfer easily between them.
So it might be possible that the last time I use iCloud keychain was for this old iMac.
Best Answer
This request comes because you have enabled Keychain for the first time with that account on this device. In order to get a copy of the current Keychain stored in the cloud, you need to authenticate yourself as the "owner" of that Keychain.
So in order to unlock the contents of your old Keychain, you need to verify that you're the same person that once uploaded that Keychain, by entering the unlock password you used on that old computer.
There's really nothing bizarre about it, nor it is any sign of a problem. It doesn't mean that your old computer is linked to your Apple-account as a device anymore - it simply means that the old Keychain data is encrypted, and you'll need the unlock password you used at the time, to get to the data.