It's a propane detector. Propane is heavy and settles to the floor. That's why it is mounted low.
Just checked the manual and it claims to detect only LP gas.
On a less practical basis, the detector in my old trailer kept going off in the middle of the night when I overnighted in a rest area between two idling 18-wheelers. I turned on the Fantastic fan and the alarm stopped. So it will, at least somewhat, detect other gasses.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10