Front Left Brake Grinding

Neither the caliper nor its piston can cause a grinding sound. They don't contact any moving parts. The two things that will make that noise are worn brake pads and rust buildup on the rotor. Rust buildup overnight or a few days is caused by humidity and that rust will wear off after a few stops, then you won't hear the noise for the rest of the day.

Deeper rust spots occur over a longer period of time and that noise will not go away after a few stops. This tends to occur on vehicles that often sit unused for months at a time, and especially if they're parked on grass. The only solution for that is to replace the rotors. The rusted areas are usually too deep for the rotors to be machined. Doing so will result in them being under the minimum legal thickness. Rust is hard on the brake lathe cutting bits too. New rotors are so cheap today that it's usually a better value to just pop on a new pair of rotors.

Thanks for the reply. But my car is used regularly. I got my rotors machined and replaced the pads together. The left side one leaves more powder on the wheel, as compared to other wheels.
The left rotor surface has developed roughness but the right one is still good. The grinding sound is only after applying brakes. I can feel the left one braking more.

Before we go any further, lock the brakes up while driving slowly on sand or gravel, then look to see that both front brakes are applying. After that, stop on a slight incline, shift to neutral, release the brakes, then see if the car creeps downhill on its own.

Feel both front wheels after driving at highway speed for a while. See if the left one is hot compared to the right one.