FLIR is looking in a narrow band-- 3200-3400nm -- where these hydrocarbons are most absorptive. @geraldmc pointed out that there are weaker absorbances around 2300nm and 1650nm, bands that might be cheaper to detect infrared in.
I took some notes on this idea after discussing and mulling it over in the 2014 barnraising, including the above info:
I explored historic single-sensor scanning concepts in the research note, but they were all full-field 2D image scanning. I think a one-line-of-pixels-at-a-time "slit scan" system could be built in a nominally DIY way (I still think it would be $1000+ in parts). A one-line slit scan system mounted on an aircraft flying in a straight line (to generate a 2D image) would be the lowest-difficulty "DIY" gasfinder, I think.