Vulnerabilities can and are usually found without code inspection. Fuzzing, reverse engineering, etc. At the same time, it is easier to find vulnerabilities having the code to check, but it is easier also for those who want to have them patched. That's why we have tons of CVEs in Windows, iOS etc., and they don't all come from the vendor... Depending on the ratio of eyeballs looking at something to fix and the ones looking at something to exploit, open source can be more secure compared to closed source.