https://en.wikipedia.org/wiki/Tert-Butylthiol - Because propane is delivered as a liquid and vaporizes to gas when it is delivered to the appliance, the vapor liquid equilibrium would substantially reduce the amount of odorant blend in the vapor.

https://en.wikipedia.org/wiki/Ethanethiol is used for propane (usually) but for the small amounts in a recirculating setup they may skip it.