This is the problem with background checks.
More than 99% of rapists get away with it (only 1.7% of reported rapes end in prosecution, and we know most women don't even report, because what's the point). Things like sexual assault, choking women during "sex", sexual harassment, flashing, voyeurism, domestic abuse, and other sex crimes, we know that 99% of perpetrators won't have been convicted or reported for any of these either, but that each perp will most likely be a serial offender. And that most offenders will have friends and colleagues in his circle who know he is a sex offender.
But no background checks will pick up on this.
What other crimes are there in society where 99% get away with it, and even though people know the guy is doing it nobody minds or says anything, so they are allowed to just carry on, escalating in seriousness, indefinitely?
We need a better way of dealing with male sex offenders.