What do you mean? why does it block it?

So you can add a test? I don't see why we care about these...

Yes, so I can add a test. This is why Special Virtual detectors were invented.

The use case for SpecialVirtual detectors is clear - we want to ask G4 to tell us about what is going on when particles pass through user-defined volumes. So for example, we want to get Geant4 to give verbose output when we are going through LH2 absorber to figure out whether particles are seeing correct physical boundaries or whatever. There might be another/better way using Geant4 interactive or verbose mode? But this is the way I know of.

isn't that what tracks are for?

I guess you want something quick for high stats.... what do you want it to spit out? same info MC hits have?

I did it with same info MC hits have. At some point I might add fields also (again, useful for debugging MC). Need also to get rid of the segmentation stuff (historical, dates back to time before TOF MC Hits)