[GrahamBrown1]

If your keeping the coolant temp low enough for the engine not to overheat, your not loosing any coolant day to day as you said you had no issues driving it carefully then pressuring the cooling system can only come from exhaust gases. A thermostat issue ie stuck closed would have the engine overheating no matter now you drove it.

Head gaskets have been known to fail on these, there is far less materiel supporting the head on these later engines compared to the first euro 3 variant. There is aLso the possibility the egr cooler could be a cause but I have never known it on them. A cracked liner has also been known however extremely rare.