Watching the seventh season, so I might be missing something, but I feel like the only way the hidden world within Supernatural remains unnoticed is because the world is populated by idiots.
I mean, the Winchesters have been caught on camera time and time again on the news, when a Doppleganger or Leviathan is posing as them committing murder. While the tape of the first “Ghostfacers” episode containing them is destroyed before it can be spread to the public, the Ghostfacers are shown to have mentioned them in their web series during that one episode where Zachariah makes Dean and Sam forget...
The crew of Supernatural loves to echo past events using imagery and dialogue, sometimes obvious, sometimes subtle. They're mile markers along the journey that encourage the audience to look back and see how far the show has come. Here's a few that I've noticed;
Pilot: The obvious mirror image of Mary and Jess both pinned to the ceiling.
Pilot: John hands baby Sammy to Dean and says "Take your brother outside as fast as you can. Don't look back."
Home: Sam says to Sari "Take your brother outside as fast as you can and don't look back." [both events happened in the same house]
Sam: That knife you had... you can kill demons with that thing? Ruby: Sure comes in handy when you have to swoop in and save the damsel in distress. Sam: Where you get it? Ruby: Skymail. Dean:Is that Humphrey? The one that needs to lay off the burgers? Ben: You know who else thinks they're awesome? Chicks! It's like hot-chick city out there! Dean: What? Someone had to teach him to kick a bully in the nads Sam: How many dying wishes are you going to get? Dean: As many as I can squeeze out. Sam: Why are you following me? Ruby: I'm interested in you. Sam: Why? Ruby: Because you're tall. I love a tall man!...