I live in the country and we have a septic tank. So I don't know about how the city sewer systems work. The last water, Mt. Dew, beet juice, etc. that goes down the drain doesn't forcefully flush down a sink or a shower drain. It kinda stays just under the rim of the drain. I think that's the way it works in the city too. It doesn't just quickly flush down into the city's system. Had he showered in either of his bathrooms his cells, DNA, would have been the last that entered the drain, it would have been near the top, of whatever remained in the drain. I've seen enough CSI shows to know that and perhaps he did too. That's why I think he used the hose in the back yard and left it running to wash away everything he washed off his body. That makes much more sense to me than his taking a shower in the house and leaving anything behind to link him to the murder. Or to track anything back outside and into his car with him.