This process is called condensation. This is what happens when water molecules in the air touch the outside of the cold cup that is exposed to air. Not much air touches the cup in the bag so not much ...
Think about what happens to the outside of a glass of ... the flow of air and essentially trapping the moisture formed during condensation against the interior panes of the window.
CRANKING your heating up during cold spells can feel nice and cosy but it can also lead to an issue in your home.
This can also happen if you put leftovers ... A little bit of condensation is completely normal in a fridge, especially during the summer months when the outside temperature is a lot warmer ...