Currently, your profile states that you work elsewhere full time. At the meet & greet: Did you discuss how their dog might do while away ? It would be helpful to know what was discussed. What did the owners say they do when they aren't home? Did they say if their dog has ever shown separation anxiety, boredom, or nervous behaviors leading to destruction of objects? It's good to know if the dog has been successfully crated, or kept in an indoor pen at home, or if they give it a chew toy or kong stuffed with treat to entertain itself or what other strategies work well?
For future stays (and actually maybe the remainder of this one) If I was going to be away from home for hours, I'd set my home up to minimize possible damages.
Aside from all that, a photo is worth 1000 words. You need to deliver this news gently, choosing words carefully, so it doesn't sound like you're blaming the dog or them. Instead, come from the standpoint of letting them know and blaming yourself. So, after you inspected it to determine that it doesn't look like the dog ingested it (fingers crossed), take a photo of the damage, let them know that rover found a way to entertain himself while you were out working and luckily (if all the pieces/shreds are there), it doesn't look like their dog ingested any, but you'll need to think of a different way for him/her to spend the time while you're away & give him/her a new project. Ask if they have any suggestions? If you didn't ask them the questions above during m&g, ask those above.
I wouldn't actually ask for reimbursement. Hopefully they'll appreciate you showing concern for their dog's well being and offer you something when they return home.