Frankly, there are many ways MT and FD could be communicating, I'm sure. If LE didn't even know MT was in CT not NY, then I hardly think LE could keep up with the many apps and secret dating sites, some which MT and FD could have used during the affair before 2017.
IMOO, NP is pushing the limits with the messaging to let State's Attorney know NP is sick of not getting any information. NP has been playing with pushing the limits from his initial dropping of the first Gone Girl and heroin comments (BTW...those disappeared quickly???).
Maybe LE is not concerned about MT because she's already cooperating or they've decided she has little to offer. However, that doesn't mesh with NP's concern as he continues to send her messages, unless poor FD is smitten and can't function without a woman in his life and keeps asking NP to contact her....his sister, his first wife, JD, MT.....He's a lonely kind of guy. I hope these aren't threats, just reminders that the doe-eyed, baby boy needs his girl.