I was keeping my eyes ahead for the most part, but suddenly the radar guard-zone alarm began squawking. A solid target has entered the pie-shaped guard zone from our port side and was closing fast. We quickly identified it as the mysterious green light, but this was no buoy. I chopped the throttle, and moments later a 20-foot center console crossed 100 feet in front of us at about 24 knots. The boat had no white all-around light and it never slowed down, even after it passed us. It was so close, we could hear the crew members talking to each other. I'm not sure they ever saw us. Afterward, we noticed that the boat made a hard left and began to run the same course as ours, but about half-mile to our starboard, its red port light indicating its position, which we tracked very carefully until first light.