I believe that once the OpenBSD team started cleaning up some of the other gross coding style stuff as part of their fork into LibreSSL that even fairly simplistic static analysis tools could spot the underlying bugs that caused heartbleed.

The bug that caused Heartbleed was extremely obvious: read a u16 out of a packet, copy that many bytes of the source packet into the reply packet. If someone put that code in front of you in isolation you would spot it instantly (if you know C). The problem --- this is hugely the case with most memory safety bugs --- is that it's buried under a mountain of OpenSSL TLS protocol handling details. You have to keep resident in your brain what all the inputs to the function are, and follow them through the code.