Coolant leak

A leaking cylinder head gasket is a good suspect. You did not list an engine size so I can only make general recommendations. Consider adding a small bottle of dark purple dye to the coolant, then check a day later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Check inside the tail pipe. If you find the dye there, suspect a head gasket.

You can find the dye at any auto parts store. Those that rent or borrow tools should have a black light.