arc1986 wrote:
My iMac was stolen and I marked it as lost, and now I found it but it’s not in my list of devices. How do I remove the lost mode?
Recover the passcode and unlock your Mac
In Find Devices on iCloud.com, click All Devices, then select the Mac that’s locked.
If you don’t see All Devices, it’s because you’ve already selected a device. Click the name of the current device in the center of the Find Devices toolbar to see the Devices list, then select a new device.
Click Unlock, then follow the onscreen instructions to verify your identity with your Apple ID password.
Once you have your passcode, enter it on your Mac.
ref: Use Lost Mode in Find Devices on iCloud.com - Apple Support