A squeal that greets you on the first few stops of a cold morning - then vanishes once the brakes warm up - is one of the most common and least worrying brake noises. It has simple, benign causes. Here is why cold brakes squeal.
Overnight surface rust
Brake rotors are bare cast iron and flash-rust from overnight moisture - see why rotors rust overnight. The first few stops scrape that thin rust film off, which can squeal or lightly grind, then goes quiet once the rotor is clean. This is completely normal and clears within the first block or two.
Cold, stiff pads and trapped moisture
Cold pad material is stiffer and can chirp until it warms to operating temperature, and moisture absorbed into the pad overnight adds to it - similar to why brakes squeak in the rain. Some pad compounds are simply more prone to a cold chirp; it says nothing about wear.
Why it stops when warm
Once a few stops have scrubbed off the rust, dried the pads, and brought everything to temperature, the noise disappears - see brake noise cold vs hot. Noise that reliably clears with warmth is the signature of harmless cold-brake squeal.
When it is not harmless
Be concerned only if the squeal continues after the brakes are warm, turns into a steady grind, or comes with a high-pitched wear-indicator squeal that does not stop - see the squealer tab. Persistent noise means inspect the pads and fit fresh pads and hardware if worn.