Slightly off topic; but does anyone have any resources that go into a higher level detail (I'm not very knowledgable of low-level programming type stuff) of how

Slightly off topic; but does anyone have any resources that go into a higher level detail (I'm not very knowledgable of low-level programming type stuff) of how an audit like the one done on TrueCrypt or a hypothetical security audit on the iOS kernel works? How can anyone know with that degree of certainty that software is secure and someone else won't find some exploitable bit?