My theories so far about what happened are:
This is WC first kill, but he likely has a history of sexual assault and violence, and I suspect LE will find some disturbing *advertiser censored* on his devices. The reason I think it's a first offence is that he's done a really sloppy job and been arrested within a week. I think he has fantasised about it for some time, and on that night, something snapped.
I truly think SE was just completely random choice and if it wasn't her, it would have been someone else. She was just in the wrong place at the wrong time. Perhaps he noticed her because she was a young, attractive woman.
I think he lured her in in an abuse of his power. I think possibly he flashed his warrant card, told her she was being followed, and offered to give her a lift home as LE.
I think LE suspected WC early on, as there can't be that many cars around in that area in that short time frame. The fact that he was LE and also did not alert them that he was in the area of a huge crime must have raised red flags.
I think around Poynders Court, where she got in, he probably destroyed her phone there and then. I think that's why there was such forensic interest in the area. I suspect he threw it down a drain, which was why they were all studying the drains there. Or there was another personal item of SE's around (earphones etc).
Given that he's LE, I think they may have found some of his DNA, which LE would have on file, on SE's phone or whatever they retrieved from Poynders Court. This then led LE to arrest him on suspicion of kidnapping later that day, when they got the results in.
I then believe LE tracked his phone to the areas where the remains were found. I do not think that WC is being compliant, because he still has not been charged with murder, and the remains were discovered late yesterday.
I think all the forensics around his properties, garage etc, is just standard protocol for a prime murder suspect. They've got to comb through anything he has to get as much evidence as possible to ensure a conviction.
EDIT: ALL IMO, JMO