Maybe the main reason that a dog gets lost is simply a gap in your garden fence and something that prevents him from coming back. It could be that your dog lost his way back, somebody has taken him away or worse – a car accident.
Another reason – when a dog is subjected to fireworks, panicked or maybe shocked by somebody, something. A terrified dog will sometimes run far away and lose his smell sense and orientation. Many dogs will be so terrified from loud fireworks that they will hide from people in strange places and even not recognize their owners and this of course can complicate recovery. A terrified dog can climb your garden fence - what normally he is not able or wanting to do.
Your dog vanished in tin air - how are you going to find it?
Remember, time is critical since a lost dog can find himself in great danger. More you postpone the search the slimmer are the chances of finding him safe.
If your home is secured by cameras check them, maybe your dog had strayed outside at night and he is still around.
Think if it is possible that somebody has stolen your dog? If it is a prodigy such a scenario is much more likely to happen. Think if somebody has a reason to take revenge on you.
If you have a gap in your garden fence, probably that's the way your dog escaped. If this is the case, then mend the gap right away before you recover your lost dog.
Look for your dog in the neighborhood, yards, parks and houses in the neighborhood where he sometimes spends his time or has a buddy he plays with during walks.
Notify your gardener, postmen, municipal workers that clean the street, post a picture on the local grocery wall, and go to the local animal shelter. Go to pet shops, maybe somebody sold him to them and you can buy him back. Go to the vet since people tend to bring lost or injured animals to him. Don't forget to leave the chip number (hopefully he has one) and a picture in all this places and check with them again and again daily or even twice a day if possible.
Regretfully, you'll have to approach road crews to see if they have, hopefully haven't, found the dog's body.
If you do not find him by this efforts, publish your dog picture in the local paper, flyers, billboards, facebook and other social media and websites specializing with lost pets with a small reward included.
For a small fee, your local post office can deliver your flyers to all the local POBoxes in you neighborhood.
Never offer a big reward, it can motivate people to scam you or ask for more and more or even worse - not willing to return such a valuable dog at all.
Do not mention all his unique identifying marks, like scars, color spots, etc. since they are essential in order to verify that the caller has actually found the real dog and is not trying to scam you or playing tricks.
Do not leave your name and address, for contact phone number and email are enough. You do not need strangers in your garden bringing strange dogs for the reward.
If your dog is friendly then we have some good news. It is easier to recover an outgoing dog than a shy one since it befriends humans and other dogs and generally wanders in the open, while a shy dog hides under cars, bushes or in garages, much more difficult to locate.
If your dog is small, it can get into some strange places.
If your dog is unneutered then it may tend to wander longer distances than neutered ones.
What can enhance the chances of finding a lost dog are a microchip and a rabies tag with your phone number.
Take in account that dogs have a highly developed sense of hearing and if your dog has some noisy toys he likes or something similar that makes a familiar noise use them when looking for him.
Put your clothes, especially out around your house - not clean clothes of course. And if by chance your dog is wandering outside near your home the familiar scent will drive him back. Take these clothes with you when you are looking outside for him, maybe he will be attracted by the smell.
When searching for your dog you'll need a powerful flashlight even if at day light, sometimes pets hide in dark places.
You can use a dog whistle to attract his attention. A dog whistle produces high-pitched sounds that are heard much better by cats and dogs than by humans. The faint high-pitched sound that humans hardly hear when the whistle is blown is too faint for humans but for dogs it's very strong and therefore it's possible to attract the attention of a dog, in our case a lost one, from far away. Dog whistles are used for dog training, especially if one wants to control his dog from further distances, than human yelling can reach, in a silent way without disturbing the neighbors.
First search in your neighborhood since the chances are higher to find him there and then enlarge gradually the search circles.
If your efforts are in vain you can always hire a professional pet detective but it could be a costly adventure.