OK, friends, I think I have here the Mystery Blood story to top all Mystery Blood stories. It took place in Oklahoma in 1932.
On three different occasions that August, J.T. Gilstrap and his teenage son Andrew shot at prowlers vandalizing the family farm near Wyandotte. After all these encounters, large trails of blood led from the place where the Gilstraps saw the intruders. However, officers found not one trace of any wounded man or men. It also puzzled them that miscreants would continue to come back to the same farm after the attempted thefts gained wide local publicity--not to mention after being shot.
This already odd case plunged head-first into The Weird when samples of the blood found around the Gilstrap farm were sent to the state chemists in Oklahoma City. The scientists reported that the blood they had been given to analyze was not human.
This left local authorities completely baffled by what to do next. The last word I have found on this story stated that the Deputy Sheriff consulted with the County Attorney and they "agreed there was little to be done, although the mystery is not much nearer solution."