Don't second guess yourself about having been able to spot this very early. Cats are very stoic, masters of disguise, even if they are in discomfort until they can't take it any longer or something becomes noticeable. The cat I mentioned had no symptoms aside from a slight swelling on her face that looked like an impacted tooth or the like; even the vet thought that was what it was going to be. Cancers start years before they are noticed even in imaging and most of the time our immune systems can fight them. A dx of cancer always seems "sudden" when it is not. You probably missed absolutely nothing early on.