Hydrogen Sulfide doesn’t smell much like t-butyl-mercaptan which is the most common odorant for natural gas. At least not to me. And not to most people I think. However, the way to test would be to start the car in the driveway and let it idle in PARK for ten or twenty minutes. If you get the smell, then suspect the catalytic converter. If you don’t, get in the car, step on the brake and wait. If you then get the smell, suspect an overheating electronic component associated with the brake.