The first was gone a month. We suspected she had hitched a ride on a moving truck at the neighbors house, got the route, and were even looking in the other part of town. She simply showed up on our porch a month later, skinny and a bit freaked out.
The second was a porch stray, a grouchy old man who decided humans might be ok after all. He disappeared in late winter, after acting a bit weird, like suddenly he didnt want to come in for a few days. Then he showed up 2 weeks ago, talking up a storm, a tad skinny, and definitely acting like nothing had changed. He became all lap-ish, though. Id guessing someone several blocks away made him an indoor kitty, and maybe passed on.
If you cant find a body, dont give up. Ive had a friend lose their dog for weeks because a neighbor swiped it and hid it inside her house. But she had to let it out sometimes to poop and another neighbor busted her and spilled the beans. Ive lost a cat to a neighbor down the block and been fine with it, as the cat decided she had more goodies to offer. A different neighbor lost a cat to us for the same reason, and they were fine with it. All sorts of weird stuff happens, good and bad, but many kitties who are gone for a long time do come back, or get spotted, if theyre otherwise ok.