Over at the (Peter) Bishop house, Peter's plugged the Observer bullet into a bunch of meters and gauges and power sources and whatever else and then he stands back and waits. After a moment, the blue pulse starts throbbing along the spiral cut around the length of the bullet. Peter reaches forward to touch the thing -- please remind me again what a genius he is? -- and then reacts as if he's been electrocuted when the top shoots a beam of light to the ceiling.
Peter goes racing upstairs, because somehow he knows that the light is projecting through the ceiling without burning a hole in the floor. Good thing he does, too, because standing there in the beam of light is September. "Thank you," he says, as the light vanishes and he's left standing there on the floor (I guess it would have been tricky to deposit him on the tip of the bullet). Peter asks what September is thanking him for, and September says, "They hid the universe from me. They locked me out." Now, September, I think maybe you could try a little harder at work too. "What I led you to, is a beacon. I needed you to activate it, so I could find my way back," he says.
Peter's all, "So I helped you." He looks kind of pissed. Clearly he's thinking, "Suuure, when an Observer needs to find his way back, all you have to do is turn on a cosmic flashlight, but still I'm stuck here." At any rate, he wants September to help him now, because he's tried everything. "And nobody here can help me, but you can. I know you can." Peter says "please" and everything!
September cocks his head, displaying about as much confusion as it's possible for any Observer to show. "You have been home all along," he says. Peter doesn't understand, what with him having been erased and all. "There is no scientific explanation," says September, in a colossal understatement, "but I have a theory: Based on a uniquely [sic] human principle, I believe you could not be fully erased because the people who care about you would not let you go. And you would not let them go."