This is exactly what happened over here, I thoroughly cleaned the shims and then lubed them. Rears started to squeak, then the fronts, that said it happens ~50% of the time during low speed braking.
If a re-bed doesn't solve it I'm going to use copper anti-seize on the shims after another bath in cleaner. Will snap some photos too.
I do take some comfort hearing that, even though we know its not the best answer. If I was the only one I would be questioning my skills but its a straightforward job so I don't believe its user error. What really gets me is why only 50% of the time? How can it be completely silent one stop sign, but not the next. Heavy highway breaking has no issues. Its been a good enigma trying to figure it out.
Since i'm not alone, it gives the thought that its just the pads and rotors getting used to each other, or a some kind or micro-particles in the pads chirping. Time will tell, if it doesn't subside after a few more thousand miles, I may redo the persistent squeaky ones.