It could even be something that isn't admissible, or isn't strong proof.
I was reading No Stone Unturned - the story about Necrosearch, as they came up in the Morphew case where there is also no body. That book is about the multi-year efforts to find missing bodies in murder cases. I was kind of struck by 2 things.
First, it can be very difficult to find dumped bodies, even when they are not especially well hidden, if you don't know where to look for them. I think that is the case here, but they were hoping once they had the suspect, they'd get a break to where the body might be.
Second, often police can be well aware of who did it, but quite far from proving it where there is no body. So it might not have taken very strong evidence at all for it to become clear to HCW who the killer is - especially when they found more circumstantial evidence relatively quickly. In one case in No Stone Unturned, law enforcement knew a body had been dumped in water, due to tips from associates - but finding exactly where took years until they got the right break (again a human witness).
So going back to the original post I responded to, I think HCW may believe he has a good idea what happened (matches what he has said) but is some way from a solid case - especially without the body.