I'm not sure if I am missing something here,
I can't imagine being in this situation, as it wouldn't be acceptable to how I want to share my life,
But I would ask him to open the door, switch on the computer and let me look at it, if he didn't or resisted in anyway, I would end the relationship,
I would find it totally unacceptable to be made to feel like this justified or not, I would find that been put in a position where I was suspicious of someone in my life totally unacceptable, that alone would end a relationship for me.
no one when making a life plan, adds, Husband will make me feel insecure, and suspicious, by locking a part of our home, and has a computer I am not allowed to use.
chances are you are not the next Sonia Sutcliffe, but the man has gone to a lot of effort to keep something secret, so you have to decide,
ostrich approach, carry on and live with it,
Confront and deal with the reaction, will he immediately give you full access without any delays?
or do you assume he is up to no good, open that door, and take the computer to an expert who will be able to tell you, what your husband has been up to, if he objects after you do this, you explain that is what happens when someone behaves in a way that causes so much concern.