To consolidate some comments I had made on this.
Somebody there had to have known what happened that night.
This means that somebody must have known at the time it happened.
Somebody there must know what happened later that night.
This means that somebody must know now.
The choice of tense is a matter of personal choice. There's no reason why must know couldn't be used—but there's also no reason why it must be used.
However, do note that if it's actually talking about somebody knowing in the present, either there would be dropped or it would be somebody who was there. It's unlikely that the person who was there that night is still there.
Consider what happens when you replace know with see:
Somebody there had to have seen what happened that night.
This expresses the fact that it's not really possible for somebody to have not seen what happened the night it happened. In this case, the detective needs to find that person (the one who saw it) it and question them.
Although the specific verb is different, the verb tense in had to have known is being used in the same way. You can also think of had to have known as had to have been aware of.