Yes, LE would have definitely known after accessing his data, but that might be something they don't reveal to family, considering the delicate nature of the investigation. Warning the public would be complicated as well. I mean what would they say "careful if you're using ABC app, there's a user named ChickenPants we think might be a killer". I know that sounds silly but really, what else would they say, without letting ChickenPants know that they know, he or she was the last person Alan responded to that day? I've never actually considered his killer might be someone known to him, and that's even more tragic, really.