> One of the biggest parts of the Free and Open Source Software definitions is the freedom to study a program and modify it; in other words, access to editable source code.
You don't have to have FLOSS-compatible open source license to run security audits on the code. For instance: Microsoft allowed government entities to check Windows security-related source code for many years. Just having access to the code is enough for audits, regardless of the license.
> One such reason is that source code is necessary to have any degree of transparency into how a piece of software operates, and is therefore necessary to determine if it is at all secure or trustworthy. Although security through obscurity is certainly not a robust measure...
If code is not open sourced it doesn't mean security through obscurity is employed. It simply means there's no public access to the code. This is a very common misconception.