The tools gitleaks and trufflehog can catch secret keys before they're checked in in git, but they don't get triggered by secrets in notebooks.
Theoretically it would be a perfect fit for nbQA to also run (one of) these tools, once the notebooks are already converted to py files.
The only issue I can expect is that you don't install gitleaks with pip but would typically use with a container.
Related issue on the gitleaks-side, missing notebook support: gitleaks/gitleaks#914