I'd lean toward the thermostat. It sounds like it may be getting warmed up and opening up in the city driving, then staying stuck open once you're on the highway.
Depending on conditions (primarily speed and engine RPM secondarily ambient temp) your coolant can be more efficient than the car wants.
If you're driving street and staying between 1500-3500 RPM, and at speeds below 50 km/h (or 30 mph... I'm American so excuse slight conversion mistakes ;) ), let's say that you get a cooling value of 5.
Now if you go 100 km/h you'd double the amount of air through your radiator and get a cooling value of 10. On a lot of vehicles, going 100 km/h in the final drive gear will leave you at a lower RPM if you're just cruising. (My wife's car for example sit around 2500 cruising at ~60 mph.) This of course means you motor is producing less heat, and getting double the cooling via air.
Normally your thermostat will close-up a bit to keep the motor in range, but if it stays open, it can very easily cause a significant drop in engine temp. So I'd start by checking that out. As a note, it may pass the usual boiling test, but just kinda stick open a little bit. So it could be very difficult to test without throwing a new one in.