In fact, sometimes! Sometimes I check GitHub's difference between tags and open commits with commit summaries too vague on the first page. Yeah, it's pretty much just a basic scan, but since it's open source I don't think they'll be brave enough to do things too nefarious and hidden, which like the bystander effect you've said, I feel like will come to light anyway. Hell, someone somehow managed to find furry porn in Thorium.