My car keeps loosing antifreeze?

I have a 1999 Oldsmobile Alero 2.4 litter Twin Cam.....every time the temp outside gets below 0degrees (im from Minnesota--this happens alot!) I loose all my antifreeze---the reserve tank is bone dry---there a no leaks that I can see, i believe the oil is clean, and no residue on the tailpipe...i have no clue as to where its going--neither does anyone else that Ive asked including two mechanics