I think Noah is involved in some way. I think the message that Audrey got at the police station hints at this. Noah leaves the room and while he is gone she gets a message saying (something along the lines of) "I know what you did last summer.... get it?" and I feel like Noah would be the kind of person to make references like that since he is into horror/slasher movies and stuff.
Also, Audrey scrambles to hide the phone as Noah walks in (to possibly see her reaction to the message???) and then yells at him. When she goes and apologises he she is like "Walk me through this murder board again" (again I'm not sure of the actual quote) and Noah points to some old messages the stalker has sent and says something like "this is the last communication you have had with them right?" and I honestly feel like he said that to work out if he could trust Audrey and see if she would tell him the truth about the new message or not.
It seems to be a weird question since Audrey went to him with the original messages. Why would he question if she got anymore? Why wouldn't he just trust that she would tell him about it? I think Noah is definitely up to something.