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What besides stones in the brake pads can cause high pitch squeaks? Noise is off and on, doesn't matter what travelling speed and is present with no changes regardless of whether the brake or accelerator is being used or not. Non 4WD car.

You don't say whether it's front or rear or what type of vehicle, but as you mention pads, I'm assuming you've identified the wheel, and it has disc brakes on the suspect wheel. I'm also assuming it's rotational and changes with speed (as it would with a stone trapped in the disc, which it seems you've eliminated).
There are numerous possibilities. If you jack up the vehicle, is it possible to spin the wheel and hear the noise? It's possible the backing plate may be bent, rubbing on the disc or maybe something else is rubbing. If it has discs which incorporate the handbrake (iie; handbrake shoes) maybe there's something amiss there or handbrake cable etc rubbing. Also, does the vehicle have original wheels and tyres, original suspension height? You may need to get your machanic to check it out if none of these suggestions seem evident.