1999 Honda Civic engine coolant

Ou have an external leak or a leaking cylinder head gasket. If you don't see wet spots under the car, you can verify a leaking head gasket by adding a small bottle of dark purple dye to the coolant, then search 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. If the head gasket is leaking, you'll find the dye inside the tail pipe.