Sleeping in a new spot, grooming less, missing the litter box — the changes owners apologise for mentioning are usually the useful ones.
Cats evolved to conceal weakness and they are very good at it. By the time a cat limps, the problem has usually been there for months. What shows earlier is a change in pattern: a cat who stops jumping to the windowsill and starts using the chair, a coat that dulls along the spine because turning to groom it hurts, a cat who sleeps somewhere warmer or lower than usual.
None of those are dramatic and all of them are worth a phone call. Bring a short list of what changed and when — it is better data than anything we can collect in a consult room.