My mama always said what's done in the dark comes to the light and that's so true.I think if you are at a point where you feel the need to go through their phone then you already know something is up or don't trust them.
I have never been through a phone and I feel like if something is going on usually they will tell on themselves anyway so no real point.
Trust your gut. I've ignored mine before and had shit blow up in my face. I am still not going through your phone-it's only going to hurt me more.