I meant maybe he's... trying to get "HOME" home... as in to wherever that was when he last lived with you? Or maybe he ran off... to... ugh... .... some animals.... do this to kind of die.
Not sure if you have heard of this... but if it really is unusual for your kitty to do that... then some pets, retreat and kind of disappear on purpose, when they know they might be dying. It is a known behaviour for animals, like cats and dogs... for some to do this.
I have personally taken it to maybe be... like... a sort of behaviour that maybe they don't want to freak anybody out, so kind of go where no one will know to do their final business. Didn't want to sound grim, but somehow my post went that direction... but I know you are scared.. it's just that you said that you come by everyday...
But, as odd as that sounds, there is that other possibility that maybe your cat is somewhere... and an even odder possibility that maybe somebody grabbed it... honestly I have been tempted to grab people's cats that are often outside... but this is more out of concern and partially out of anger, because winter comes, and they don't damn let their cats back inside... and ... something could happen to the cat.
Can't tell you how many times I have been tempted to take a cat in as my own, but the only thing stopping me, was that a family member is allergic... it is possible that somebody felt the same way with your cat... and thought that nobody who owned it cared (if they constantly saw it wandering around outside), or maybe they saw it alone... and thought maybe it's homeless so took it in.
Also... though posters are out... most people drive around... so they don't really stop that much to look at things, because they aren't on the sidewalk.. and pass by stuff... so... it's possible people still haven't seen it.
But there are a few possibilities here...
Another is that your cat has ran off to a place beyond where the posters are.....