I've posted here about my wheels squeaking before, but I'm pretty sure it isn't due to the brakes as I've had them checked recently and even regreased. I don't have much knowledge on car maintenance but could it be the plate behind the brake disc touching as it spins? The squeak seems to disappear when the brakes are pressed and at higher speeds than 20mph. I've heard getting a screw driver and bending the plate back a few mm could fix it. Is this worth looking trying?
I could be some rubbing from the stone guard but that wouldn't change when you press the pedal. Only way forward really is to get under/in there and investigate it. Jack it up, spin the wheel and try and hunt down the issue.
My money is still on something brake related though. If you are sure the sliders have been cleaned and greased properly then it could be caliper piston. Also worth checking the anti squeal shims to make sure they have some slip on them.