My mustang typically does this when things get cold/wet and it sits for a bit. No visible signs of rust, but a bit of squeel, which goes away after the brakes have heat cycles a bit.
My best guess is that temperature causes some weird expansion/compression somewhere, that is fixed after heat cycling. I have not noticed this type of thing on my SHO, but it has been a while since it has sat outside in any type of adverse condition.