Hi Mate
I had very same with my scoob, the rads are very prone to splitting on the plastic end tanks, only leak under pressure and mine was very hard to find as it was evaporating.
I took rad out and checked the end tanks/caps found crack about 2 inches long.
I was thinking the worst, thought it had to be head gasket as no water was visibly leaking.
Give the rad a really good check both ends.