Must have been garaged and only driven on sunny days!
The sunroof drains in such a way that the rust is hidden at first - rusts from the inside out - hopefully yours is not affected.
Can't speak from experience, as I've never had a sunroof - but I believe the Micra has a slightly undeserved reputation for rust, mainly from the sunroof issue.
I bet if you compared non-sunroof K11's with other cars of the same age, you would find them no better or worse than average - if they have been looked after well, they should be fine.