There are loads of places where you can have a password, on a phone, a computer and so on, so it would help if you told us WHAT password.
And that is the point of a password, to stop people seeing what you are doing.
But to be honest, I think that is the wrong way to go about it.
If you think he is having an affair then just come out and ask him.
You will be able to tell by his reaction if he is.
If he says he is NOT then you can say that he is acting like he is.
(This is getting more like it belongs in Body and Soul)