It is narrated entirely in the past tense. So, if I were to put that sentence in the present tense, I would say that Theon has yet to find that secret way out, but if he does, he wouldn't trust it. Now, in the past tense, it is:
"Even if he found some secret way out, Theon would not have trusted it."
Theon had yet to find a secret way out, but if he found one, he wouldn't have trusted it. Why is it "have trusted" instead of "trust"? Because the act of "not trusting" that way out isn't continuous, ongoing or habitual; instead, it has a beginning and an end. So, let me try to simplify it: Theon doesn't trust the door, so he goes on and forgets about it.
I hope that helped.