[fliprayzin240sx]

Buddy of mine with an S13 redtop have the same issue. His shit was leaking on the turbo water line at the back. Basically, when the car is hot and the cooling system is pressurized, thats the only time it would slowly leak. Since the car only gets that hot when he's driving, he never see any coolant leaking. He just notice that his radiator is low ever couple of weeks.

I'm not saying its leaking from the same area but you get the idea of what could be happening.

Also, is your reservoir overfilled? Sometimes if you have a shitty rad cap, itll let coolant out into the reservoir but not let it back in when its cold.