
The only time
ever that I experienced anything like this was several years back. But not just one dog, nearly all of the dogs in the house were sneezing.
My vet couldn't find the source, but put all my dogs on something for allergies. It all came to light when I found an extremely old and musty, broken open sack of birds peanuts that had lain for a couple of years amongst some bin liners in the garage. Probably, if I had ever put the car in the garage, I would have noticed sooner.
Once it had been broken open by one of the dogs, it seems like most of them got their heads in the sack. It was all dusty and musty and horrible.