First of all, if your pet was having diarrhea, the staff and doctors would have contacted you and started them on medication. That's the good thing about boarding your dog or cat at a veterinary hospital - if they develop problems, the issues are addressed immediately.
Secondly, pets often experience excitement when returning home and are often rewarded with treats. So this “excitement” leads to colitis, a common cause of loose or watery stool. Some pet owners expect it and we arrange to send them home with preventative medicine.
We take every precaution at Academy East Veterinary Hospital to prevent viruses and parasites from being passed to your pet. Stalls, runs and cages are sterilized. We also require dogs to be checked every 6 months for parasites (unlike annually at most vet hospitals). This is the reason: some of the parasites we see are easily spread between dogs (Coccidia and Giardia) and are not prevented by monthly heartworm preventatives. We have staff that clean/scoop fecal material as soon as it hits the ground.
To recap, loose stool or diarrhea is not uncommon but please let us know if it continues for more than 24 hours once your pet returns home.