Greta just reported this and I think Devlin must have saw Ben and he scoped him out when taking Shawn to see his friend. So, this leads me to believe how he knew about Ben.
It could be that Devlin dropped Shawn off and went cruising around the area while Shawn visited with Devlin and that is when he spotted Ben.
The info I got came from KTVI 9pm news. I have to replay it again to find all the details exactly. That will be later tonight and after the 10pm news
Oh ok I see. Wow, how creepy and ick. I wonder how long he was on the look out for the "one".