when I apply my brakes, they sometimes make squeaky sounds. they also sometimes makes this sound when I'm taking a curve, or even when I'm riding without even engaging the brakes.

a week ago, I opened the back fluid reservoir and found lots of dirt in the fluid.

the pads are not worn out

fairly common occurrence

if in a dusty atmosphere brake fluid absorbs water and dust ....the fluid should be changed regularly (yearly ?)  in any case ....as the piston works in the master cylinders there can be some rubber dissolving into the fluid discolouring it ...as the rubber ages and wears from the dust contamination

sintered pads seem to make the most noise from my experience ....the organic pads mat not work as aggressively as the sintered but less noise and wear of the disk overall ...

squeaks around corners  could mean that the wheel bearings are  starting to wear out ....or the forks out of alignment vertically ...which could be fork oil needs replacing ? (getting some twisting effect on the main axle - forks .) or disks warped slightly

just some ideas to explore ?

I actually pass through a road construction daily. 

I will make time to check the areas you have pointed out, I just pray its nothing serious.