It will be coming out of the overflows because of the overheating. The cause of the overheating is more likely to be a thermostat or water pump issue. You said you had been topping up so I ruled out coolant shortage as the problem, plus, you will need to pretty much drain the system to check the components anyway so buy some proper coolant to replace it with after. You could also flush it if you feel the need while you're there.
Cheers,
Jen.