Leaking antifreeze

There are dozens of places coolant can leak from. The thermostat housing is by far one of the least common places to leak. You need to search for the exact location of the leak. A pressure test may make it easier to locate. For really slow leaks, you can add a small bottle of dark purple dye to the coolant, then search a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. Auto parts stores will have the dye for the fluid being checked, and those that rent or borrow tools should have a black light.