An open source software project like XCSoar is in a good position to do this, because the developers are only paid in kudos, glory, and self-satisfaction. (There is no revenue stream to
maintain).
I think self-satisfaction is the primary stimulus for most open-source developers. This would also explain why they keep adding features: only when the developer himself encounters a serious
defect he will be truely motivated to fix it.

There are proven techniques for finding hidden defects, for example
Some of these practices are performed by the XC Soar team as far as I know (from hanging out on their IRC channel).

1)Code inspection
This is done extensively. I have seen _a lot_ of discussion regarding code quality, performance, architecture, etc.

2)Functional testing
XC Soar does have unit tests, if that's what you mean.

3)Exhaustive model-based automated testing.
As far as I know this is still mostly academic. When I graduated two years ago, a lot of research was still performed.
The tools I worked with mostly did code generation, using a (verified) model as the source. Now, when you already have an extensive codebase (like XCSoar), reverse-engineering that model is hard
I think.