I too have had the same anomaly and I don't know why but I have a theory. (Guess) As the cold weather sets in the seals on the calipers get cold and shrink up as well as less resilient. As it gets colder and colder the brake fluid in the lines and calipers contracts. There is now a slight void and vacuum internal the caliper and lines. The caliper seals are stiff and do not seal real well so the system draws in a very insignificant amount of air. You wouldn't even know it if you rode the next day. Problem is the bike does not get used. The temp goes up and down every day and every week. Eventually all the insignificant amounts of air combine to become significant over months.
I would think that the fluid in the master should run down to replace the void but I wonder if the small hole in the master reservoir and thick viscosity from the bitter cold makes it easier to pull the air past the caliper seal. Just my best guess. What do you think?
I have another theory..............
This happened to my GL1500 during warm weather. I could tie the brake pedal down for a day and then have almost a full pedal. One week later the brake pedal was back down all the way.
The only permanent solution was to replace the pistons and seals. That corrected the issue.
Tenp changes had nothing to do with my issue..........!!!
A similar situation occurred with my friends 1996 GL1500.