Rust on downtubes?

[sportchuck]

I usually apply a coat of wd-40 to the metal/chrome on my bike once the first snow hits as that's when I stop riding. This year I got some winter gear for the holidas so didn't do that hoping I get one day of clear streats well that didn't happen. So I went into the shed to day to do the winterizing. Everthing seemed good maybe just some dirt except it looked like there was some rust begining to form at the top of the fork sliders (or downtubes I get the two confused at any rate the area on the forks between the triple tree. Strange though as there are no marks at all on the tubes below the clamps? I put some wd on a rag and proceeded to wipe it did pretty much nothing came off until I used polish or my nail to scratch it off. I try to take real good care of my bike. Wash and wax 3 times a year only cuz I like to ride more than wash and winterize every year just a little late this year. Wouldn't think a couple months in the shed would do this my bro's v-rod is right next to mine and other than dirt not a spot on it. The shed is a metal one on a raised wood floor and waterproofed. There is some condesation in the warm months when it rains but only on the tank and only occasionally.
No pitting or anything this was just weird anyone else ever seen this. And what if anything can I do to get rid of it and back to new. Got rid of most of it but don't want this repeating. The bike is a 2006 FXSTi. Also as a side note to keep the moisture level to a minimun I use damprid and buckets of rocksalt.