In my world, there has been a zombie apocalypse at some point in the past. Humanity got hit hard by it, and the number of humans has been reduced to a few thousands when it happened. Humanity managed to actually win against the zombies over time, since there was a huge but limited number of them, and they are pretty dumb.
Now the human population is back within the millions (it's been decades, maybe even centuries, I haven't decided yet). We are rebuilding civilization slowly but surely.
Obviously, zombies didn't fight among themselves or eat each other (that would make any sort of horde impossible, therefore making them no dangerous at all). They aren't particularly interested in eating brains, but do love eating humans. They would probably eat other animals in they managed to grab them. Only humans can be turned into zombies.
They act basically like in The Walking Dead as for moving slowly, going after survivors by sound, sight and smell, but you need to have had contact with a zombie (bite, scratch, etc) to turn into one. You don't turn if you die from other causes, but your dead body can be turned later on if it gets scratched or bitten.
Now from a realistic perspective, even though they are technically dead already, zombies move. They need bones, muscles and energy to do so, even if they are almost brain dead, and don't seem to feel pain or bleed out from cut limbs. A human that gets turned probably has calories to keep moving for a few days, maybe weeks if he has lots of fat. Catching something and eating it will obviously result in a calorie intake (they can diggest you assuming they still have the necessary organs, and this can probably be handwaved if they don't).
Let's say that zombies show up on day 1, and reduce the world population through kills and turning them into zombies to a few thousands within 5 years. How long can I expect zombies to stick around after that, assuming humans are now well organized and almost nobody gets turned anymore? I'm looking for the very maximum length of time that there could still be a few zombies around, and what exactly did those outlier zombies do in order to last that long.
For example, if the majority of zombies died within 1 year, almost all of them are dead after 3 years, and maybe a few dozen of them can still be found after 5 years. What did those last ones do differently to last so much longer? Did they happen to be in a specific location, or were "asleep", etc. Where did they find the energy to live so much longer, and of course having an idea of how long this would be is good, but that's not the main question.
In this context, I want to set my story up so that everyone knows about the zombie apocalypse, but do not expect to find zombies anymore, but end up finding one or a few of them at some point, which take them completely by surprise (unexpected), but is still possible.