My ex snooped on me, didn't trust me, eventually I started doing what he suspected me of

It's weird, it became a self-fulfilled prophesy. Him I hid shit from, because I felt like he was invading my privacy and didn't trust me, loooooonnnnng before I ever
thought about (much less did) anything that warranted him feeling that way.
My current husband and I have a basically open door policy, and the fact that we
have such a policy is what drives the fact neither of us would ever dream of going through those doors (or drawers, or internet caches or what have you's) if that makes sense.
So, to get back to the real crux of your question, i.e., when does snooping cross the line? I think snooping crosses the line the minute someone feels compelled to snoop constantly. Because that compulsion to snoop is the red light something is wrong. Of course, the question then becomes is that something purely a problem of the person who wants to snoop (insecurity) or an actual
relationship problem (the small still voice that whispers "they're cheating on you," being a witchy person I do not discount the powers of intuition, particularly coupled with those of perhaps subconscious deductive reasoning)?