The pond, maybe? That's be the easiest solution, since it's a body of water right there, and you wouldn't be able to figure out that there was blood in it unless there was basically a body dumped in there, right?

[ He shrugs ]

It's just a thought, but why not compare the bruises to our suspects' hands? We can't prove anything about the blood, but that's quick and easy. It's not much time wasted to see if the bruises match up to anyone's fingers.