It's harder than is obvious, the odorant must spread in the same way that the hydrogen does under wind and buoyancy. Natural gas is a bit of a heavier molecule

It's harder than is obvious, the odorant must spread in the same way that the hydrogen does under wind and buoyancy. Natural gas is a bit of a heavier molecule but hydrogen is very light and I suspect that the hydrogen and the smell would very rapidly separate. A bigger problem is that hydrogen can leak through tiny cracks, and odorant can't.