Fork breather?

[rustyrider]

I've ordered a set of fork seals so will be changing them as soon as they arrive and see if that cures it.  I'm intrigued that I can ride it and nothing gets past the fork seals, yet if I leave it sitting on the sidestand somewhere hot, it pisses past the seals.  I've pulled the top dust seal out and it is definitely coming past the seal, just not when I'm riding it, only when it's parked up.