I'd like to bring up the color yellow.
Most obviously we've got David's t-shirt in the latest episode with the yellow triangle, giving a visual indication that the YED is emerging/taking hold.
In the opening scene of the series, baby David is laying in his crib, on yellow sheets. Perhaps a hint that David has been infected with the YED since infancy, as Carey speculated.
And in the opening scene of episode four, we meet Oliver Bird, whose outfit is all kinds of yellow. Which, along with the weird speech he gives that's clearly a reference to how this story is playing out, gives a big signal that he's got more to do with this story that we yet know.
Which brings me to another point about Oliver's integration into the story. We know that the YED may have been with David his whole life, 30 years, and that Oliver Bird was lost to the astral plane 21 years ago.
When Carey asks David to think of a really stressful memory, right before David and Syd project to D3, David starts to think about trick-or-treating as a child (maybe even 8-9 years old). Something terrifying happened to David that night, something the YED did. Maybe it has to do with Oliver Bird and is the same night he got trapped in the AP.
This is not the first time Halloween comes up either. David ends up on Amy's doorstep during trick-or-treating after he leaves Clockworks. Don't know what anything I've said means, but in shows like this, repetition isn't coincidence.