Thermostat failing won’t cause coolant loss. It will lead to overheating if it doesn’t open or slow engine heating if it’s stuck open. The best way to find a coolant leak is with a dye kit.

Also, post what your car and engine are so we can have more info. Or put it in your profile.

__________________
2005 X5 4.4i SterlingGrau, Sport Package, DSP
Build 3/12/2005