Define "smoke." White smoke -- steam, typical of a coolant leak or condensation from cold exhaust system -- or blue or black smoke, typical of an oil leak in the engine or turbo bearings? At 100K miles, I'd suspect turbo bearings are just starting to show some wear.

I'd also suspect that your missing coolant has nothing to do with a head gasket. These cars are notorious for going through heater cores -- I've replaced two so far, meaning I'm on my third core at only 80k miles. Fortunately, they're cheap ($160 at fcpgroton.com) and fairly easy to replace.

Another thing to check out is the upper and lower radiator hose clamps. My upper hose clamp started to leak at about 60k miles, and it took me a while to find that slow leak before I replaced it with a $2 clamp. The lower clamp just went at 80k miles, but at this time I know that the OEM clamps are starting to fail on these cars, so I replaced the remaining three clamps on the two hoses and I have no more leaks.

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Jim Rothe, '99 S70 T5M,
http://www.jimrothe.com/volvo/index.html