In the quake2 days, there was a setting called gl_modulate which allowed you to adjust the lightmap to insane levels -- essentially making it full bright and removing any strategic elements offered by shadows in the map (not to metion making the maps look like total crap). It has been removed in the standalone, but the server-side check is still there, so if people are using the quake2 client and set their gl_modulate really high, it makes them glow. If they can see you in the dark, then you can see them in the dark, basically. It's not a hack, but rather a mechanism for making the game fair when I didn't have the engine source code available to disable the built-in "cheats."
