Twice last winter I had to cut rides really short due to almost complete loss of front brake – boths times it turned out that brake fluid had squeezed past the seals. Both days were sub-zero.
Googling showed this has happpened to other riders in cold weather.
I cleaned everything and replaced the pads both times – I'm still using the same seals with no further problems.
So I think shimano seals are adversley affected by the ambient temperature, perhaps they pull back the pistons better when its a warm day, but get more brittle/rigid when its cold, thus leaving our pads rub-rub-rubbing away at the disc.