It can be a number of things, from slight misalignment of certain components, to very thin deposits on the rotors, to random debris jammed somewhere in the brakes, to the pads compound simply being noisy, etc.
Unless you feel vibrations in the pedal when you brake or you feel sudden changes in pedal feel and brake power, then I wouldn't worry too much about it.
Sometimes shaving a bit of the leading edge (and possibly even the inner and outer diameter edges) of the brake pads off is enough for most common noises to go away.
Some pads have a built-in system that warns the driver when they are too worn, and they do that with a little metal flap scraping on the rotor surface, making lots of noise. I doubt that's the case if two mechanics already inspected the car and told you the brakes are fine.
There are many things you can do to try and fix the noise: You can change pads and have your rotors properly checked for deposits or uneven wear (which is almost always invisible to the naked eye) and then have then resurfaced or replaced as needed. You can have all the moving parts lubed and the rest of the system cleaned from dust and debris.
You can do that now or wait until you wear down the pads enough to replace them.
Squeaking isn't dangerous, just annoying, so don't worry too much about it.