We call it a "propane" detector but it will actually detect any flammable/explosive gas (butane-propellant for spray cans, hydrogen-charging battery, methane-cows/dogs/old men, even gasses from new carpet and glues can set it off) so it may not be propane that is setting it off. It could have simply been some other gas sitting stagnant in the AC ducting that was stirred up when the AC was turned on. There is no propane directly associated with the AC system or ducting.
2013 F150 Lariat Supercrew 3.5 Ecoboost 4x4
2017 Springdale 220BHWE
Retired US Navy Master Chief, Nuclear trained, Submarine qualified