There was blood on them, it was late and washing machines are noisy.
WHF is a large, period house with many rooms and was built to stand for centuries. It has three staircases and walls around one foot thick. I believe the washing machine was in or near the back kitchen ( a sort of storeroom and utility area) and well away from the living and sleeping areas. No one would have been disturbed by the sound of that washing machine, in my opinion.
This suggests, doesn't it, that there were reasons other than the noise of the washing machine for soaking those clothes in a bucket or buckets of water rather than just popping them into the machine and washing them?
Everyone hates having buckets of clothes soaking in their kitchen or storeroom, they're a mess and accidents waiting to happen. The sole reason anyone soaks clothes is because they have to, because these clothes are so badly engorged with staining that they require pre-soaking.
As AE stated that the clothes smelled of blood and soaking is solely used to remove staining, this has to be the answer: those clothes were sufficiently stained - with blood - to require pre-soaking to loosen the stains.
The clothes were also so badly stained that AE could detect the smell of blood on them even after the blood was highly diluted by soaking.
Perhaps AE and the officer could even see that blood because the water in the bucket/s was tinged red or straw coloured by it?