If there are no visible leaks, the two likely places to look are the heater core and the engine itself. Check your carpet for moisture. Are you getting any odor inside the car? If this isn't the problem, the leak is probably in the engine and it is getting burned. Do you get any white smoke from the exhaust?

 

Your water pump is leaking, I have 191K miles and mine was always needing fluid in the reservoir. It was when I blew the small hose coming out of the radiator back to the engine, that I found the telltale signs of a leak coming from the water pump. The water pump has a little hole in it that when the water pressure gets to high it lets out water there, well as you can guess after time it starts letting water out all the time. Another thing to look at that happened to me is the little pipe that connects your two upper hoses has corroded and is leaking, that happened to me also and you can only get that part from Nissan. I paid alittle over $10 for mine.

 

John, check the small bypass hose located in the front of the engine on top. It starts at the water neck where youadd coolant and curves directly down. It is not a high pressure line so it won't gush out coolant, but it will leak. However it won't be visible because the coolant drips into the plastic scuff plate under the engine compartment that is curved enough to hold a cup or two of coolant. When you drive, the coolant "sloshes" out the back of the plate and out the back of the car so you never see it.Obviously it happened to me, and drove me nuts! Hope this helps.

 

Mine did the exact same thing as Steve's.