Brake Squeak Noise... HElp me...

[03whitex5]

about an week ago "check brake lining" warning came on with RED brake sign.
i went to body shop and asked what's going on.
they took of my wheels and found my pads has only about 2mm left.
but i couldn't replace it rightaway because of the time.
so i went back to shop after a week later, with BMW syntec oil and Sensors to do it all together.
he did change oil and brake pads ( he said original bmw part ) but not the sensors he told me i can reuse it.
so we did the just pad and oil change job.
and yes, red brake light still staying but brake job seemed good to me.
and he promised that he will replace the sensors next time for free.
but the problem started about 10minutes after i left the shop.
i was heading to the toll lowering my speed and at about 20mph it was sqeaking, i didn't know if it was from my car because from high speed more than 30mph i couldn't hear any squeaks if i hit the brake slowly.
but when i came back to my neighbor and hit the brake at low speed as 10~30 i hear really loud squeaks, i was so shocked and afraid what the hell is this sound, so called shop rightaway to ask if it's normal ?
he said it should be gone soon. is it right?
please help me someone.....
how do i avoid this damn annoying very loud sound, and is red brake light will be gone after sensors replaced?

oh yea I did research of this forum and found about bedding!!
let's say i just run my baby for about 500 miles is there any chance this noise will go away?
cause i usually drive about 100miles a day.