Could be something really simple like condensation from the cold weather.
Check coolant levels to see if you are losing any. If so then it could be a sign of hgf but don't jump to conclusions.
Check the oil cap for any bad signs to rule it out. If coolant is leaking into the oil it will be quite obvious as it will be a milkshake colour inside the cap. Also check the dipstick for the same.
Check the coolant reservoir for coolant loss and also if oil is present there.
Could be something less sinister though like a bad oxygen sensor. If this sensor is faulty then the ecu can cause over fuelling which would throw the emissions off due to it running rich.
Check the easy things first and go from there. If you are unsure what to look for ask a mate who knows their stuff before spending money at a garage.