We need to learn why there was no evidence? I'm sorry. Not following that. Sometimes there just isn't anything that can help solve the case. LE did all they could do. They weren't just "waiting" around. There is no magic way to predict what some secret psychopath is planning unless they actually tell LE or someone else does.
This is the real world. Not a tv show like CSI which has, IMO, led to the attitude that if the cops don't find evidence or solve it within an hour, they didn't know what they were doing.
Nope. Most of these crimes can't be predicted or prevented. Sometimes perps are good at not leaving DNA or other clues.
That's reality.
It's a needle in a haystack issue. If a person doesn't have a criminal record, isn't a sex offender, hasn't done or said anything that others are able to notice that would be obvious enough for them to suspect he's hiding a live person in his house, etc., there's nothing that can be done and nothing LE needs to learn so they can do "better" the next time.