Thankfully, there's an army of people devoted to tracking them down. Pet journalist and blogger Rachel Spencer speaks to them and gets their tips to find your lost pet, fast.
As a nation of devoted pet parents, we all want to keep our animals safe. But plenty of them go missing every year. Rachel Spencer from the Paw Post spoke with Tom Watkins of Animal Search UK and Jayne Hayes of DogLost for their tips on how to get your furry friend home safe and sound.
If your pet is missing, contact a service like Animal Search UK or DogLost. Both organisations understand the process and know exactly what to do, and have tens of thousands of volunteers in the UK to help.
First, they create a profile with the pet's name, age, breed, colour, distinguishing features, photos, microchip details and the owner's phone number – plus printable posters and flyers.
'Owners then circulate them in their neighbourhood, at the vets, shops, stations, as many places as possible,' says Jayne. 'Not everyone is online and other dog owners are the people most likely to spot a dog.
'That's why we urge people to use posters, particularly if a dog is stolen, because friends, neighbours, even the postman will see them.'
DogLost has 50 regional coordinators who compile lists of contact details and addresses for local rescues, vets, councils and pounds in their area.
'We suggest owners call every day and tick each one off, and visit to drop off a poster,' says Jayne.
'Plenty of people who find a dog or cat don't take it to the vet,' says Tom. 'They call and report it, meaning there isn't the chance to scan the microchip.
'Ring and ideally visit all the places you think your pet might turn up, and make them aware your pet is missing.'
It became compulsory for dogs to be microchipped in the UK in April 2016.
While it's not a legal requirement for cats, it's highly recommended as it is the quickest way for a vet or rescue to find the owner when a pet's handed in.
'If your pet is missing, ring Petlog who register your pet's microchip,' says Tom. 'Make sure they have the right details, especially if you've changed phone number or address since owning your pet.'
Contact your pet insurance provider, as they may help with costs to find your pet. For example, LV= will reimburse you advertising costs if you're looking for your missing pet, and may also help fund a reward.
'This is something I would urge owners to do beforehand,' says Tom. 'Insure your pet with a company that offers advertising and reward cover, so you're prepared should the worst happen.'
It's better to share the post created by the pet tracing service, rather than relying on your own, to maximise the number of people who see it.
'An appeal on your own Facebook may only reach friends,' says Jayne. 'It will understandably be highly emotional and may not contain the information people need.
'By sharing the posts search organisations create, you're reaching tens of thousands of people committed to finding pets.'
Walk around the area where you last saw your pet.
'Search at dawn when it's quiet,' says Tom. 'Take a friend or relative with you and carry a torch and a cat basket, or your dog's lead and collar.'
7. Leave a smell they'll recognise at key places
A missing pet may be unwell or confused, so finding their way home could be a struggle. They'll search for you with their sense of smell.
'Put an item of clothing or a tea towel you've rubbed on your skin by the front and back door,' says Tom. 'It's a smell that they'll recognise and may help them identify home.'
Dogs will often return to the spot where they last saw you.
'Keep looking and listening,' says Jayne. 'Put up a poster where they went missing and leave something that smells familiar, like a blanket or toy.'
'Keep everyone in the family positive and never give up hope,' says Tom.
Jayne agrees. 'We've reunited people with dogs who were missing for eight years,' she says.
Tom's Missing Pet Search Team success rate is 80 per cent, and Jayne estimates she's reunited 80,000 pets with their owners in 15 years.
We spoke to two owners who followed these steps and were reunited with their dogs.
Zee, a five-year-old Belgian and German Shepherd cross, pulled his lead from Samantha's hand while out walking in Clocaenog Forest near their home in Wales.
She contacted DogLost and shared their appeal on Facebook. Her local community rallied to help, with a drone firm offering a free search for Zee.
For four days, Samantha, husband Paul and children Isabella and Solomon searched for him and finally heard him whimpering. His lead was wrapped around a tree.
'He was cold, thirsty and hungry, but thankfully was soon back to his old self,' says Samantha.
Karen was visiting her mother near Croydon on Boxing Day 2017 with Jack Russell and Corgi cross Bailey.
The 11-year-old pooch was let off his lead on a walk and bolted, but was far away from where he lives in Hartfield, East Sussex, and couldn't find his way home.
Karen contacted DogLost and shared his appeal on Facebook. The following morning she had a message from a friend telling her to look at the DogLost found page.
'Bailey had been picked up in a different postcode,' says Karen. 'The dog warden dropped him off at my mum's. I was so relieved to have him home.'
For more information, follow DogLost on Twitter @DoglostUK, or find them on their Facebook page. You can follow Animal Trust UK on Twitter @AnimalSearchUK, or find them on Facebook page. Follow Rachel Spencer on Twitter @rachelspenceruk.