Correction - we don't know a ton about these people. Just what the BIL has publicly shared. OTOH, LE definitely knows a lot by now and I expect they will come out and do a news conference of sorts pretty soon to give an update on the case. Isn't that typical in these types of cases?
We absolutely do know a ton comparatively to so many other cases. These were not private people by any stretch of the imagination. Could they have had private matters that only some knew about? Yes and that's the portion that we don't know but I guarantee you that anyone who matters, meaning authorities who will officially solve this, know all of it by now.
This wasn't a private childless couple who didn't socialize. They were the exact opposite. If something that could've been risky, whether intentional or not, was going on, it's going to be learned.
People these days are all about themselves. Attention. Breaking the news. I have never seen more people pop up and say "I knew them" or "I have connections" in public forums than I have in this case. If this was done by someone connected to them in any way, and I don't see how it could be the opposite, it's going to be known fast. People love talking, people love being in the center of "hot" news and people will volunteer that info quickly.
Couple that with technology of this age. There's no way on earth that the seconds of a person walking down in alley is all that was caught on camera. Various people have pointed out the many cameras in the area that are visible with a quick Google search. That's not including any that aren't readily obvious. The shuffling person with the skinny legs is simply all they're throwing you so that you'll have something to go on. Just like the people who will get their attention-needs fed by volunteering info, the authorities would be ALL OVER the news to get attention considering this has become a national story. They aren't. They're silent. Why? Because solving something like this is the BEST press that they can get.