In mixed rural/urban areas such as where I live, you will occasionally see stations advertising that they sell off road gasoline. It is marked by a red dye. The fine is very hefty if you get caught driving a standard vehicle on a public road without the marked fuel.
I'm sure it still happens occasionally, but the incentives to snitch are so good, you will get caught if it happens consistently.
Ditto for natural gas. The rotten egg smell from traditional natural gas piped into your home is a marker or additive for safety reasons. Unmarked natural gas is odorless and colorless. If you have a road approved natural gas, it is easy enough to add a new odor or color at the pump. A consumable filter which treats x cubic feet of natural gas would probably be the way to go. Once the filter needs changing, you have to replace it to maintain the marker.
I think it would also be possible to keep the pump from operating once the filter was no longer adding the marker. A lot of this stuff can be controlled with a 70 cent microchip.
The Japanese are into their second decade of doing exactly this. They started putting natural gas in buses, taxis and other forms of public transportation when we lived there in the late 1990's. Now, it is ubiquitous and working its way into the consumer market.
I live in a NG producing area. Marker odor is all over as we have vast underground storage fields also, but whatever...you’re right. I’m wrong. Have a nice day.