I have never heard of brake fluid freezing unless it had water in it.
My guess is the brake pad light is telling you exactly what it is supposed to tell you: You front pads need to be replaced.
It is not uncommon for someone to visually inspect the front pads, delcare that they still have life in them, and then a short while later have the light come on.
Why? Because the sensor is embedded roughly 1/16" from the brake pad backing pad. This is so you are warned before the metal brake pad backing starts making contact with the brake disk (rotor) ruining them but most importantly loosing your braking from that wheel.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)