It could be going a variety of places. How much coolant are you losing/day?
My dad had a cracked head on our Ranger and it sucked about a gallon every two weeks. No smoke, no problems with the oil, and no bubbles.
If the cap were bad, it would go into the overflow tank. Where are you losing it from? If it's the overflow tank, make sure that's not leaking too.
My only other idea is a bad heater core. Can you smell antifreeze in the cabin when you turn the heater on? If so, you have a bad heater core.