My Thermal Camera Looks Different At Night Than In The Day?

I was told thermal cameras cannot see light so why does the image look different in the dark at night than in the day?

Temperature differentiation. If you have significant sky or water in the scene and have not configured around it is also a possibility. True Thermal Imagera do not see in any visible spectrum to create an image. However evenings are usually cooler. There are a host of other possibilities such as dew on the lens.