Turn off the propane and see if it still happens. With no source at all, I'd suspect a faulty detector. Low cost, home-type CO and propane detection haven't advanced at the same rate and efficiency as smoke detection, so the technology isn't as advanced. Because the technology isn't as accurate, the devices are by default designed to be prone to false positives rather than no activation when ther's product present.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter