Well, it wouldn't be the internal fork spring, as that's bathed in oil. It also should not be the fork seal, because on reassembly after replacing the fork seal or doing any spring work, it is recommended to lube the top of the exposed sliders with fork oil or special grease if you installed HyperPro springs. If you didn't lube your fork sliders and some road grit got in there I suppose it could squeak. What about any rubbing parts, as in the brake hose? Is it secured properly?
Were the fork springs reinstalled properly, with the tighter spaced coils at the top and the looser spacing of coils at the bottom? Did you remember to put the spring seat back in on top of the spring before reinstalling the spacer?
Just asking, as these might cause squeaking. Other than that, I'm as stumped as you. Maybe it's something else really simple that we've all overlooked?