At the rate you are loosing coolant, it could easily be evaporating before it forms a puddle. Take a close look at your radiator and where the rubber hoses connect to the different fittings. Look for scale and crust build up or the tell tale sign of a green stain. I have seen on older cars where the alluminum hose nipples, like where the thermostat connection is, will build up oxidation underneath the hose and lift it away from the metal enough to cause it to leak. I think you can also by a UV dye that you can add to the coolant.

TimK

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EPA 609 & 608 HVAC Certified
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83' 300D Turbo - 300K + Miles
00' 1.9L TDI Beetle
85' 6.5L Turbo, GV-OD, GMC Suburban,
67' 2.5L Diesel NA - Land Rover Series IIA

Here's what you do if it's so cold your diesel fuel gels. Smear some on some toast and stay inside until it warms up outside.