Handbook or a dealership should know what it is, but I suspect
that you have correctly identified it.
Some pads have a plastic bush with a stud and connector, as
the pad wears the pads backplate with bush and stud move
towards the disc, when they meet a circuit is completed to
ground and the warning light comes on.
With wet salty gritted roads it could be that it's sufficiently
conductive to trigger the light, I'd wait until it stops raining.
Failing that I'd hose off any muck in that area and let it dry,
brakes and pads are pretty water resistant, but it's not
really the weather for hoses at the moment.