Boiling radiator fluid means too high a temp. This is generally caused by running too lean, lugging the engine, riding with the wind, dirty air filter, and water pump impellar damage or failure. If your bike was fine before the problem, look to the impellar/air filter. If it is after a rebuild or new part addition, look to the jetting.
More than likely the fluid loss is from the over temp and dumping fluid out the overflow when riding and you not noticing it. The lower level of fluid is from boiling it out, not neccesarily a leak in the cylinder or rad cap failure.
As noted before, pin holes in the rads will lower the fluid levels without being very obvious, especially if they have built up dirt on them. If you don't "keep it clean", it's hard to catch the little stuff before it gets big. Also look to the radiator tabs that mount to the frame or shrouds. The heavy vibration of the KX5 and laying down the bike will cause some stress on these areas. A leak may start there as well.
Sly