A review of the low-cost but very useful Honeywell EZ Sense gas detector / sniffer, which can alert you to escaping propane, butane, LNG, LPG and natural gas.
About the cheapest place I found this for sale is here (alas, I do *not* get a commission from this!)
www.safelincs.co.uk/honeywell-ezsense-flammable-gas-detector/