Marking is often confused with a housetraining accidents. Most dogs and even cats, will attempt to mark in a new place. It's a dominance behavior. A perfectly housetrained dog will still try to mark. They want to make the new area theirs. A host home must be as free as possible of other dog's accident/marking scents. That's why carpets are preferred pee spots. It's harder to completely remove the scent. It's there even if the human nose can't smell it. Carpets and rugs need to be deep cleaned often with a pet odor eliminator as well as any other spots that seem to be pee magnets.
A car ride usually jangles a dog around enough to have to eliminate as soon as they arrive so dogs immediately go outside. The owner will be in the middle of telling me, " oh they went out right before we left" and the dog starts pooping.
I do not allow dogs that are untrained unless they are puppies and I do not allow potty pads. A dog that sneak pees or poos in the middle of the night or while I'm not looking would be crated for sleep or while I showered if it happened more than once and I would tell the owner. Face it, we cannot have our floors and carpets professionally cleaned between every guest. They need to know what their dogs do when they aren't around. I explain to them that it is perfectly natural for pets to exhibit unusual behavior when not in their care. I go over all of those things during the meet and greet or in messages if it is an out of town client.
Neutered dogs don't mark as much as unneutered dogs so I don't accept un neutered dogs. The biggest complaint I hear from new clients about why they didn't return to a past sitter is bad smells in the home so we really have to keep up on that at all times. I always have a mop and bucket ready for immediate clean ups and a hot water carpet cleaner to run once a week.