firmware password reset.
how to reset firmware password on IMac
iMac, OS X 10.11
You can only remove the Firmware Lock with the password that was originally used to set the lock. That MacBook was probably lost or stolen by the original owner. Firmware Locks are a common place with a device is issued by an employer to protect proprietary data in the even the device is lost or stolen.
Axel F.
Is there any way to reset firmware lock - Apple Community
Firmware password protection in an Intel-based Mac - Apple Support