Hidden Cue was a small feature added by developer in patch 2.0.1, due to small percentage of people actually beating the game. It's been already discovered, after hints that it might be visual cue.
|Reveal the secret|
The loading bar's background color will remain blue, if site does contain the Red Room URL part (key). But only for few seconds. It doesn't hint if it's on site's subpages, so you need to browse through these too.