Well, I also know from experience just because a person has a background check done it doesn't really mean anything.
I know of an instance in a church where someone was a volunteer with the youth, had a background check done, nothing was found, the reason being is they just hadn't been caught--it was found out later they were molesting their own child..they hadn't touched any other children..but that didn't matter..so yes, sometimes, the background checks can be misleading..and give a false sense of security..
But at the same time, it has also been a deterrent for others when they know they will do a back ground check..we had someone come in to apply for a position at our church, and when they were told we do back ground checks they left..they went to a different church and applied--and hired, they didn't do a back ground check..someone who went to the church recognized the person as someone who had been on the news a few months earlier..for molesting children at a local daycare (the former daycare had been closed down, and the person was out of jail on bond)