My MacBook Pro suddenly has activation lock on it from an Apple ID I don't know

It sounds like it wasn't prepared properly for you. Theft discouragement features are still set. Get that account owner to do the things in this article. --> What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

The people on this forum do not work for Apple and cannot speak for them, but from what Apple and others users say, your options are to either have the account owner remove their information, or if you have the original receipt for the computer, ask Apple to reset it.

● Read this article to learn how to remove Activation Lock either on the device itself or on the web --> How to remove Activation Lock - Apple Support

or this one for disabling Find My if activation lock has not yet engaged --> Turn off Find My on your iPhone or other devices - Apple Support

or if the owner no longer has access to a device it can can be removed from Find My on iCloud.com. as described in this support article Remove a device from Find Devices on iCloud.com - Apple Support --> "You can use Find Devices on iCloud.com to temporarily remove a device from the Devices list or turn off Activation Lock on a device you already gave away or sold."

● Help with a lost Apple ID or password:

• For help with a forgotten Apple ID (this is likely an active email address or one used in the past), click here --> If you forgot your Apple ID - Apple Support

• For help with a forgotten Apple ID password, click here --> If you forgot your Apple ID password - Apple Support

● Reset device if you can prove to Apple's satisfaction you are the owner:

Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. This is done online* at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not provide many details in the article as to what is acceptable proof. Prior to this official service Apple typically required the original receipt** issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the first owner who does this, or a descendant with legal proof of ownership transfer). Note that while this will let you get use of the device again, it will not help you with lost access to an account.

* There is a special link for this service. To save time you should use it instead of https://getsupport.apple.com/ .

**Apple recommends you keep your receipt after you buy an Apple device. --> Keep your receipt after you buy an Apple device - Apple Support

You're right, Apple does not say that outright. It is more of a passive implication in the Activation Lock + Find My feature. In theory if a device can be rendered useless to any buyer unless prepared properly, it is less attractive as an item to steal. That's not to say that if you see Activation Lock that an item is necessarily stolen. It just means that the item is not ready for resale. It is something you need to confirm before purchasing any used equipment. Unless somebody actually sets a device in Lost Mode remotely to activate Activation Lock, a device may continue to function for a while until you do something such as try resetting it.

Find My and Activation Lock is fairly recent on Mac computers. Activation Lock for Mac - Apple Support