My car has also developed a very slight "squeak" upon startup. It only lasts for less than a couple of minutes.
My truck, on the other hand, has been squeaking on and off for the last 20 years! And I have never been able to find a sure-fire fix for it!
Sometimes, there is debris caught in the ribs of a pulley (or pulleys) that can be a cause. The same goes for the belt. Also, if there is any sort of liquid getting on the belt, it will squeak.
I personally think that the serpentine belts are more prone to squeaking than the V-belts were.