See the article Check your Apple ID device list to see where you're signed in - Apple Support
Permanently remove a device from your account page
If you're still signed in with your Apple ID on the device, it might reappear on your account page the next time your device connects to the Internet. To permanently remove the device from your account page, sign out of iCloud, iTunes & App Stores, iMessage, FaceTime, and Game Center on that device, or erase that device.
When find my phone was enabled the activation lock will automatically turned as on .
An admin account having strong password , will stop any one accessing the Mac .
Your Apple ID and password , the account credentials are with you , and you didn't revealed to any one .
From iPhone see the article Use Lost Mode in Find My iPhone on iCloud.com - Apple Support
Note : you had not signed out Apple ID and password from Mac system preferences , so it will show in iCloud ( as the Mac was lost ) , if others who are now having your Mac will not be able to open Mac an admin password is created , even they try to https://support.apple.com/en-us/HT212190 , they can't access iCloud data , activation lock stops to access the account Activation Lock and Find My on Mac - Apple Support
Note : arrange another Mac and sign in with Apple ID and password with account credentials and see if your old Mac appears in the list of devices Check your Apple ID device list to see where you're signed in - Apple Support
