I have realized that in G4 10.3 (but the same is true also in G4 10.1.p03, so I guess it is a long-standing problem) Geant4 runs without crashing, and without any error message, when one or both of the following environmental variables: G4LEVELGAMMADATA G4RADIOACTIVEDATA are defined but point to a wrong place. In other words, currently there is only the check that these environmental variables are not null strings. Instead, there should be a check that they point to the correct place (as it is correctly the case for the other environmental variables). When any of the two above environmental variables point to an incorrect place, Geant4 should crash immediately (as it does correctly for the other environmental variables).
The check for G4LEVELGAMMADATA is committed to the repository. It is not obvious how to add a check for G4RADIOACTIVEDATA, because no data are uploaded at initialisation. Needs clarification from the code authors.
I'll have a look at the radioactive decay model.
Added check in G4RadioactiveDecay that G4RADIOACTIVEDATA is pointing to the right directory.