OK, time for the degreed chemist to chime in.
Hydrocarbons are a class of organic materials that contain mostly hydrogen bonded to carbon.
Alkanes are a type of hydrocarbon that only contain hydrogen and carbon. Light weight alkane hydrocarbons (methane, ethane, propane, butane) are usually in the gaseous state at room temperature and pressure. Higher weight alkane hydrocarbons (pentane, hexane, etc) exist in the liquid state at room temperature and pressure.
Alcohols are a type of hydrocarbon that contain an OH group attached to one or more of the carbon atom(s) such as methanol, ethanol, etc. and exist as liquids at room temperature and pressure.
Gasoline is a mixture of many different hydrocarbons existing in the liquid state.
The tail pipe sniffer draws in exhaust gases for analysis. The hydrocarbon detector will detect all hydrocarbons present in the exhaust whether they are from unburned fuel (now in a vaporized state) or are the byproducts of incomplete gas combustion. This type of detector is designed and calibrated to detect small amounts of hydrocarbons measured as parts-per-million (ppm).
Hydrocarbons in the liquid state are measured by a different method and are usually measured as volume (liter, gallon) or by weight (grams, kilograms, ounces, pounds).