What signs tell you a parrot isn’t feeling well, even if it seems okay?

I’m trying to learn more about the early signs of sickness in parrots.

They hide discomfort so well, and I worry I might miss something important.

What should I be looking out for, even when they seem fine?

Has your bird ever acted normal but turned out to be sick?
What were the small changes you noticed?

I’d really appreciate any tips or stories. Just want to be more prepared.