So if you hold the thermometer outside the bag, in the shade, it reads the same temperature as inside the bag with sun shining on the bag? Maybe so, but I don't see how that would be possible unless there's air flowing through the bag when you're moving.

Yeah when I worked at Starbucks we had a bunch of those mechanical thermometers, and every so often I would stick them in a cup of ice water and adjust them all to read 32°F exactly.