I've had this problem with all of my cars that have front disks and also 4 wheel disks. If you get moisture or rain/snow etc and leave the car parked for many hours overnight for example, the brake rotors will develop a very light patchwork of rust/oxydation. When you first apply the brakes you will hear a squeeling noise or a grinding noise, which is the thin layer of rust/oxydation being rubbed off the rotors by the pads. Should go away after a few applications of the brakes. If it doesn't, it means that the pads have been glazed by excessive heat/braking during break-in etc and the only cure, I believe, is to have the pads replaced and possibly the rotors turned. Which of course, means you have to break them in again---no hard stops etc...

My maxima brakes used to make a very loud noise after rain or after getting wet until the heat dried them out.

And also, if the pads are excessively worn, they will chirp each time you apply the brakes to let you know its time to have them changed.