I've had the same, also been driving for an hour and everything ok. But first thing I noticed was temperature light came on. Pulled over and stopped. But already lots of steam and coolant all gone.

​​​​​​It started with a crack in a coolinghose.
Turned out head gasket was gone to. Small engine revision and replaced all hoses and new waterpump and gaskets to offcourse.

​​​​​​Hopefully it's just the waterpump for you.

greets