I don't think they noticed the blood the first day, necessarily. But they assigned homocide to the case within a few days of WC having gone missing, probably because they found blood.
In this article it's stated that the police were there every day for about a week, looking for clues. The press conference came shortly after, which is when we the public found out about the blood.
https://signalscv.com/2017/01/31/homicide-detectives-ramp-probe-cierzan-case/
There's a lot in the article that describes WC's personality, including that he has a photographic memory. I wonder if he heard or saw something that caused him to become a threat to someone else. Remember, the nephew worked at 6 Flags for a short time, got into a fight and was fired. I wonder if WC learned something damming about the nephew from other coworkers, and he confronted the nephew.
I'm also feeling that WC was "escorted out" after he was struck while still inside the house. He may have even been partially bound. Just guessing, that's not stated anywhere.
In multiple early articles, it states WC last spoke to LC AT 5:00 pm the day he disappeared, and she herself said she DID NOT KNOW the nephew was there that day. This matters, because I can't imagine there was enough time to kill WC, carry him out, clean the blood and other evidence in those 6 minutes. Maybe it's possible, but I find it unlikely.