Visitors Story:
I noticed that over the course of my relationship, my fiancee started to become very uptight with his phone. He literally started sleeping with phone in the bed next to him, taking the phone with him everywhere he went, and never letting me look through his phone.
Even on the rare occasion when I did look through his phone, he would have erased all text messages and dailed/received calls. This was a major red flag! If he wasn't cheating(which later I found out he was)he wouldn't have to be so over-protective with his phone. This is a red flag that everyone should keep an eye out for.