If it was the bat, this would make a bit of sense. After all, whoever has it presumably has found a very good place to hide it, so they could simply return it to its hiding spot and not necessarily even need to worry about cleaning it up. We haven't found it yet, after all.

[ He shrugs ]

Otherwise, my only guess would be the chains themselves, since the water could have washed away the blood, but even then, I don't think that's the case. We would have seen traces in the water, I would imagine... At least considering the sheer amount of blood.