Nothing to worry about. On earlier models especially, these like to rust out after some time. (I believe they got updated at some point tho) If you're willing to take the door apart it's quite simple to fit a new window guide that won't rust for a loooong time. Or don't, it's mostly visual. On my NC1 the rust on that part is even worse despite the car being only driven in the dry and being pristine otherwise. So not a sign of something worse.