Ditto what Deb said. This dog probably doesn't have accidents at home, because the owners have shown her where to go and not go at home. But now she's in a new place, where no one has shown her where to go and not go. She goes on the familiar potty pad, but she doesn't have the experience to know that your carpet isn't also an acceptable place. She's allowed to go on the pad, but she's also allowed to go tons of other places, like in the yard or anywhere on walks. Dogs don't generalize concepts in the same way humans do, so when you're showing a dog "go here, but not here; here, but not here," they're learning those specific places rather than extrapolating that information to mean outdoors is good and indoors is bad. That generalization is also being complicated with the use of the pads, so indoors is also good sometimes! And on top of all that, she's under a fair bit of stress being in a new place with different people and her family gone. Essentially, even in the best case scenario, it takes a lot of repetitions in new environments before a dog will begin to recognize the broad pattern of behavior you want instead of just individual rules in specific locations.
There are definitely dogs out there who can go into a new environment and immediately know what's up with the potty situation, but they tend to be dogs who have had a lot of experience in new environments and who generalize skills quickly. I'd say easily half the dogs I watch have at least one accident in the house, and I only watch house trained dogs. It's just part of the business. The dogs who have more difficulties just need more supervision, more frequent potty breaks, and a little effort from me to teach them where they're allowed to go in this new place.