They are known for bad WP seals on the 1.8, the SLLC pink coolant seems to seep out of them (my mom’s ‘06 Matrix does), but it would have to get pretty low to start air locking the heater core. Throw a new quality radiator cap on it too, the xB in my sig lost coolant due to evaporation due to a leaky one. Or rent-a-tool a combustion gas coolant tester & get ready for a head gasket replacement…