Leave a car parked for a few days - or a few weeks - and the first drive often brings grinding, scraping, or grabby brakes that were perfectly quiet before. This is one of the most common and most harmless brake situations. Here is what is happening.
Surface rust forms fast
Brake rotors are bare cast iron, and iron begins to rust within hours of exposure to moisture. While parked, a thin film of surface rust forms on the rotor faces - see why rotors rust overnight. The longer and damper the sit, the thicker the film. That rust is what you hear grinding off during the first few stops.
Why it grinds and grabs
As the pads scrape the rust off, you get a grinding or scraping noise and sometimes a grabby, uneven feel because the rusted surface grips differently than clean metal. After a handful of moderate stops the pads polish the rotor back to bare, smooth iron and the noise disappears - usually within the first mile or two of driving.
Longer sits and pad sticking
After a very long sit (weeks or months), rust can also lightly bond the pads to the rotor or the parking-brake shoes to the drum, causing a clunk on the first roll as they break free. Heavier rust may leave faint pits, which can cause temporary noise or pulsation that also cleans up with driving - similar to cold vs hot brake noise.
When it is not just rust
If grinding persists well after the brakes are warm and you have driven a few miles, or you feel it steadily through the pedal, that is not sitting-rust - it points to worn-out pads; see why brakes grind. Inspect the pads and replace them with quality pads and fresh hardware if they are low.