I'm guessing - and I could be wrong - but based on the fact that its a real world thing - its likely the game where you get a bunch of people to play at hunting/assassinating each other as they go about their daily life.
The objective is to tag enemy players and become the last player standing.
I've played assassins before, which I think is what you're describing. I'm wondering if Underground Assassins is a variation on the game, or just a different name.
