They don't
(I've just checked with a friend who has related experience)
Not because it's daytime, but because they are "looking" for a blob that's a different heat. When the daytime temperature is above the mid 30s, there simply isn't enough of a difference to "see" anything. Indeed, they can be pretty unreliable with less than a 10degree difference between the background temperature and body heat, but they might try rather higher than that.
I've seen plenty of reports that show the daytime temperature is far too high to get useful results, but does anyone know how far it drops at night?