[ The graveyard is practically a second home to Spike, a place that he resorts to whenever he wants to feel a little less stuck among idiots. He also has a very good sense for shenanigans so seeing someone sneak off into that very direction, away from the festivities, carrying a load of shady shit... well, of course he has to follow.
He stalks John silently, night vision helping him keep to the shadows and a safe distance — for now. He's got some second hand experience with Janus worshippers making the most of Halloween. Who knows what this guy is up to? ]