At FL400+, anything not the earth is basically space – if you point an IR camera in the sky outwards, it really does look very cold as the pressure is basically zero and scattered IR photons are few and far between. Objects that are not vacuum have a signature in the IR – both a black-body temperature (which, even at ~ -55 ºC, is a hell of a lot higher than single-digit kelvin…) and possibly the opportunity to reflect any incident light.