What could cause 1992 Mercury Sable to leak coolant?

Simple answer would be your radiator or hoses. If you've never replaced the hoses, do it anyway. Go get a pressure test done and that'll pinpoint it if you can't. ---- NOTE:

Anti-freeze wears out and needs to be replaced. A tester costs about $2.

If you haven't changed the anti-freeze in 3-4 years, plan on changing it now.