I do a lot of PR reviews, and while I may not catch everything, I will catch a few things here and there... someone with that mindset would be in a better position to handle that from the start...
Having a few security guys that do PR reviews for about half their workload would go a long way to improving things.
We're going through an audit for an internal application now... there's 1 major flaw (SSL2/3 is enabled), a minor (session cookie isn't https only) and a couple trivial (really non-issue) concerning output caching on api resources and allowing requests with changed referrers (this can be spoofed).
In any case, having auditing earlier on and as a potential blocker would make each minor change easier to deal with than potentially much larger changes... the app in question was developed for the first 8 months without even a Pull Request check in place... by then many issues regarding code quality are already too late to fix completely. :-(