coolant level keeps going low no leaks found what could the problem be?

I too had low coolant despite there not being any coolant smell coming from under the hood, but it turned out that I had a very tiny leak in my water pump. The water pump in my 06 Solara is designed to self plug minor leaks so the leak was barely detectible. In fact, everything under the hood was dry and there was just the tiniest tell-tale spot of crusted, dry coolant. It was barely visible without a flashlight and keen eye. You may have the same problem I did, or it could be a tiny leak emanating from somewhere else. Coolant can leak from different places (hoses, seals etc.) and sometimes it is so small and slow that you won't see coolant soaking the other parts of the car or dripping on to the pavement under the car, especially after it dries. A big coolant leak is usually detectible by smell (it smells sweet, sometimes like caramel when the engine is burning a lot of coolant).