It could be a pulley. Had a guy come in and get a new belt one day (quick lube type place) and after that it sqeaked. He showed me when he put dressing on it, it stopped. Told him it's a certain pulley (it spun terribly.) He argued, saying since it spun that it was fine. He was an idiot. I'm still mad about it. Anyway. He came back and had us put a diff belt on it (thinking we put the wrong one on) and it was too tight but stopped the sqeaking. Eventually he put on a new pulley . . .
The point is, dressing will let the belt slip a little and if a pulley bearing is not spinning good, it will let the belt slip over that pulley. I've got the same problem. Had my belt shred, put a new one on. It finally developed a squeak and both tensionor pulleys, idle, water pump, and alternator pulleys are new. Tensionor might be loose, or AC, PS, or crank pulley?
But once it warms up it goes away. I'm just leaving it till it never stops.