Whatever Spike's been doing throughout the day, he's managed to miss crossing paths with Jack until at the very end of it. He's been hanging out in Sector 0 for the last couple of hours, smoking and slowly depleting the contents of a whiskey bottle. There's a few more in a backpack next to him on the grassy patch where he's made camp, necks sticking out. A closer inspection would also reveal a few bottles of blood and as many cigarette packs as he could fit. No personal affects. This isn't his first time abruptly leaving a place.
He raises his hand when he spots Jack, pushing himself back up on his feet. There's a flicker of relief, but it disappears quickly - seeming unaffected in the face of the apocalypse is kind of what he does.