Thanks for the help.
I also found this relevant topic from the old forum: https://hypernews.slac.stanford.edu/HyperNews/geant4/get/geometry/106/1/1.html
The solution is to get the top transform and perform an inverse transform on a null point :
G4ThreeVector origin(0.,0.,0.);
G4ThreeVector globalOrigin = aStep->GetPreStepPoint()->GetTouchableHandle()->GetHistory()->
GetTopTransform().Inverse().TransformPoint(origin);
Best regards,
-David