“So this crazy thing happened recently with an old Mac I sold on Craigslist a few years ago. I noticed it was still showing up in my Find My iPhone app. Well, at first I didn’t realize it was that particular Mac. I just happened to notice there was a computer I didn’t recognize in Find My iPhone called ‘Michael’s iMac’.”
This is a huge security bug. The person who sold his Mac could have locked or erased that Mac, because the buyer never signed into their iCloud account. Even though the seller had wiped the computer, for some reason, it was still linked to their iCloud account.
Whenever I sell a Mac or iOS device, removed it from my account in iCloud, after wiping it. I do that because I don’t want to see devices that I no longer have, but I do recall, when I started doing this a couple of years ago, that one or two of my Macs or iPhones were still showing up under my account.
Source: How I sold an old Mac and unknowingly had access to its location for over 3 years