Oh how terrible!

It immediately comes to mind as to wonder what the poor baby's last thoughts were, if he had time for them at all...and then to be the owner and not know what's happened to your pet...?
Do you have some kind of local shelter or rescue who comes to pick them up when they have been hit and are already gone? We usually give a call to animal control so they can come and make an identification, then inform the owners.
How very sad.

Poor pup, I hope he made it safely to the Rainbow Bridge and that his passing was quick and painless. ;_; Such a sad end!