I am leaning in agreement with luckypenny, leaving a dog in a kennel is horribly stressful.
I am sure that the kennel staff is use to dogs who get depressed and stressed during their stay, they also don't personally know the dog so they more easily write off the behavior under stress. I wouldn't be quick to jump on the kennel since they have no real way in knowing what is depression and what is a more serious illness as they can present the same. As the OP said they staff were surprised when the dog didn't react to the owner returning which tipped them off that that dog might have actually been sick and not depressed.
If all medical tests came back negative there is really nothing that can be pin-pointed on the kennel at this point.
Did the dog get any vaccines before going in the kennel that could have caused reaction?
At this point I would agree that removing the high-strength medications for some more mild ones as well as trying to feed a moist bland diet to see if anything can stay down long enough to digest.