I've had more than a few deer not leave blood trails. Fortunately they didn't go far from the shots. Guts can plug the holes real fast and leave no blood. I also shot a buck once, hard down thru the chest, from where he was standing to where the 1st blood was, was a solid 35 yds, but from then on it gushed out, and he wasn't more than 15-20 yds after that. Sometimes blood trails do weird things. THat's why you need to use asa many other factors as you can. Watching the deer, tracks, leaves kicked up, broken twigs, etc.