I also did some snooping (I now know it violates the rules) and found no hard evidence that would indicate that she is cheating (no texts, calls, etc). So if she is, she is covering it up VERY well. . . .
The other thing I noticed is that she is becoming more and more protective of her cell phone (again, no strange texts or calls though). She even takes it into the bathroom when she showers!
I suspect she's chatting via one of her apps, and not thru the text messaging of the phone itself.
I could be wrong, of course, but please keep your eyes and ears wide open.