From what I remembered when I was working and applied to a job, that (if) the job being offered was contingent of passing a drug test and also a background check. In most cases, on your application, or they could have you sign a separate agreement for a background check. Now on the form that you signed, would have the (name) of the company/address/phone to get a copy of your background check results. I do remember that the form stated I had (30) days to request a copy for free. Since I always passed, I never requested a copy.
Most fail due to lying about their education, experience, or had a prior felony/misdeamenor etc. Put simply, whatever you put on the application and answer to, is what they are verifying. Odds are, you probably know the reason and (hoped) it would not be found.