This has been going on for years. It's not just for lounges - people have been caught going airside just to see celebrities or meet friends/relatives.
Not sure what the "minimum" time is, but I've been flagged for doing a SIN-CGK-SIN turnaround, though after flashing both BPs they let me go. I wonder if they have a more sophisticated system which actually tracks whether you actually boarded the flight, or if it's purely based on time.