The Observer can be seen behind an ambulance as Walter and Astrid arrive to examine the nanite victims.

Walter: Despite their itty-bitty size, each of them is capable of storing an enormous amount of energy and it would seem they could be overloaded by the biokinetic energy stored in the cells of anyone infected.
Peter: You mean these people's movement is what's activating the nanites?
Walter: That's what I just said.

Jessica Holt: What are you?
Walter: I'm human. What are you? Is this some sort of alien invasion? Are you part of a strike f...
Jessica Holt: No, I mean--I mean, what are you doing here?

Walter: May I take a blood sample? The discomfort will only be momentary.
Jessica Holt: That's what all you men say.

William Bell: In this game, the skill one must have above all else is patience. The board changes, but very slowly. the art of chess, the art is knowing when a piece is most valuable, and then, in that very moment, being willing to sacrifice it. For in the vacuum created in the loss of what is most precious, opportunity abounds. Influence is maximized, and desire becomes destiny.

Olivia: Walter, you said that you had been having a lot of shock therapy. Do you think perhaps he could have been a hallucination?
Walter: No. No, no. My hallucinations were rarely biped and never men.

Peter: It’d be sort of like, “don’t cross the streams,” just, you know, in reverse.

In the 1984 movie Ghostbusters, the title characters use proton beams to capture ghosts. As noted, crossing the proton streams is a bad thing.

The glyph/code spells out POWERS.