For every stray cat that I have fed in the past, I'll never say good-bye. If I say good-bye, it implies I'll never see them again.
There are television news stories about pets being reunited with their owner. I think about that when it comes to each and every stray I come into contact with. Sometimes, after they are fed, some simply disappear. In my heart, though, I choose to believe that they are still out there somewhere. I'd rather think this way about them, than to consider that they might have passed on.
The life expectancy of a stray cat can be to from 2 to 5 years. That's the best case scenario. It's a hard life to be unprotected outdoors. But I have seen old cats come to be fed. They have beat the odds. That's why I won't say good-bye. There's always a chance that they have just moved to better feeding grounds or a safer location, or possibly both. I always hope for them to come back around to me.
I have had indoor cats in the past that I have shared many years with. They have passed on, but I don't say good-bye to them either. They may not be present in the flesh, but they stay in my mind...in my memories.