There is one theory about “ghosts” called “residual hauntings.” Basically, the energy from an event, usually traumatic or emotional, becomes imprinted on the surroundings somehow, and you are basically watching a replay of a “recording.” So, it could be possible that, if this took place, one would actually be seeing a Mass that took place years ago, likely a funeral or wedding. As such, it would have been valid when celebrated, but, now, it would be more like taping the Mass on EWTN and watching it later.