Interesting that the vet is stumped as well. My Otis has been sneezing and having a lot of snot lately. I can hear the congestion in his nose. I have been doing some armchair vet research on this, and learned that some cats can have allergies. You may want to pay attention to what the cat may be doing before the sneezing happens and the environment. If anything in the environment changed right before the sneezing started.