The Windows source code is the most comprehensively reverse engineered codebase in the world. Virtually every software security firm and every security product company has someone on staff who has the lay of the land for the kernel, services, and drivers. Even if Microsoft didn't publish most of the debug symbols for the OS, which they do, it'd still be the best understood closed source codebase in the world.
The likelihood of them hiding backdoors in software that you don't know about simply because some company you worked at that had access to some of their actual source code didn't have all that source code is low.
The likelihood of them hiding backdoors in software that you don't know about simply because some company you worked at that had access to some of their actual source code didn't have all that source code is low.