After it chucked it down one day last week I noticed rusty water marks on my drive near my rear wheels. Will it be from my brake discs or something more sinister?

I would go with brake discs, dont worry.

Yes, it will be from the discs, as Terry has said, nothing to worry about, that is to say as long as your hand brake works ok.

Thanks for that and luckily the marks have come off the drive so I can come out the dog house now.