One of the most important parts of understanding search methodology for HR professionals is understanding that a "nationwide" database of criminal records does not exist. Not every background check is created equal, and if you are just using a database it is extremely unreliable and risky.
A commercially available National Criminal Database should never be used alone or as the primary method of discovering records. Even if the vendor tells you they are verifying the data at the county. They are still only verifying the data they found in the database. The database itself is like a wireless provider coverage map that has holes in it everywhere. There are over 3,400 counties in the country and over 5,400 courts, and these databases include less than half of them.