Unless you have a neighbor playing tricks on you by draining a little coolant every-so-often, you DO have a leak. But, since you aren't seeing traces on the ground, the leak is probably small, and on the upper part of the cooling system over a part of the engine causing evaporation when the engine gets hot. Remember that the coolant moves when under pressure - so the "leak" won't always "drip"...the frustrating leaks occur when the pressure builds (engine is running (and therefore hot)) there is a small spray onto the hot engine and the spray evaporates, leaving very little evidence asto the problem spot.
Losing coolant with no apparent leaks, White smoke out the exhaust that has a sweet smell to it,
The radiator cap might need replacing.
How To Find & Fix Coolant Leaks http://www.aa1car.com/library/coolant_leaks.htm
You may have a bad head gasket and it may be leaking from that. Why when cold i dont know, but if you are losing coolant it may be the head gasket.
Probably the thermostat is sticking.
If there aren't any visible leaks, its leaking internally. The 3.1L V6 engines are known for bad gaskets. Is there oil in the coolant or coolant in the oil? Is there a tan frothy substance on the oil fill cap? If so, your coolant is getting into the oil.
If it had no leaks it would not be loosing coolant. So therefore it does have a leak. It can be leaking in a spot where the coolant burns away before dripping on the ground. Also a blown head gasket is a possibility. Look for coolant in the oil. A heater core can leak. Do you smell coolant inside the car or are the windows fogging up or developing a film. You need to have a pressure test run on the cooling system and find this leak.
The coolant leaks out.
It leaks coolant.
Only if the sensor is malfunctioning.
If you are losing coolant, which would cause overheating, you need to check for leaks. If the coolant is fine, the next step is to replace your thermostadt (cheap.) If that doesn't work, either you got a bad thermostadt, or you need a mechanic to check it out. If you haven't noticed any leaks on the ground, or steam gushing up, chances are it's the thermo--assuming you have coolant in the radiator. Check that first--on a COLD engine, not after it's hot!
It sounds like a loose hose clamp or hose leak but it could be your rad. it self. leaking