Until now, locking record mechanism in Salesforce only can be initiated from Approval process. Related to this, some users able to unlock locked record by click a button to unlock record, so it would be editable. Users with that permission will see a 'Unlock Record' button next to Edit button.
Compare the screenshot when view by record owner or other user as below.
If you notice above screenshot and the one above it, user with permission to unlock record will also have permission to approve / reject a submitted record.
So, who is the user will see the Unlock Record button and able to unlock the record? They are:
1. User with profile System Administrator
2. User with Modify All Data permission
3. User with Modify All permission in the object locked
Once a record is unlock:
1. Lock icon will be disappear
2. Record owner or users in higher role hierarchy will able to edit it
3. In the object history, it will show date/time, user and action 'Record unlocked.'
Reference: Salesforce: Edit Locked Record