I'd say you are missing the concept of remote temperature sensing versus direct contact temperature sensing.
They are not just working with temperature (as in body heat) but with infrared radiation (the other form of temperature). So the covering material must be transparent to this wavelength. Depending on the sensor type used the wavelength will be different, but as humans are roughly 30°C on the outside, their main wavelength is somewhere in the long wavelength infrared region (~10µm), so sensors are used which are very sensitive in that region.
A typical sensor uses the pyroelectricity of materials. As those sensors are only able to detect changes, they often have special fresnel lenses built in such way, that you get stripes of detection areas. A moving object which crosses those stripes will then generate several pulses which can be measured.
So you have to use a material which is transparent at 10µm wavelength. Typical lenses are made out of polyethylene which gives it that whiteish/milky look. There are coloured lenses around to make them look better (more fitting to the case), though some have reduced transmission.
If you want to get through ABS you have to change your wavelength. Radar sensors work fine through ABS as demonstrated by TURCK for example.