Mmmh, the same happened to me recently with my S270. I also suspect that I have some high frets up the neck (I haven't used the straight edge, but I can see it in plain sight, some frets are flat as an ironing board!). It did happen relatively suddenly too, and some strings are worse than others.
I would imagine that the fret can 'suddenly become high' if:
1. It has become somewhat loose
2. You tend to play a lot from frets 10-20 (which I do) and fretwear has finally appeared . I imagine this can happen somewhat suddenly: the difference between buzz or no buzz can be milimetric
3. There has been a change in temperature or humidity and it affected your truss rod.
Have you tried your truss rod? I have managed to change from unbearable fretbuzz in the upper frets to almost unbearable buzz all over the board
Well, I don't even know what I'm talking about. I'm no expert, so don't pay much attention: I just have the same problem and I'm trying to work it out. If memory serves, I think there's a pretty good tutorial on fret dressing at projectguitar.com (where you use a marker to colour your frets before sanding them to ensure they're even when you finish)
Steve Vai walks 'the fine line between pagan and christian'. I say walking the fine line between fretbuzz and playability is a much harder one!
Hope that helps