IMO, on the backpack topic, anyone who bought the backpack at a Walmart will be on camera and will be well documented. And the video quality is quite good as well. The trouble is which backpack is "the" backpack. This could account for part of the "thousands of hours of video" they mention.
Whether it's a backpack or a candy bar, Walmart will know:
Where it was bought (which stores sold one)
When (date and time sold)
Which checkout aisle was used (which leads to which camera to pull video from the relevant time)
The cashier on duty (whether self checkout or not)
Whether cash or card was used to pay
If cash, amount paid and change given
Anything else included in the order
Video would not only show the person buying but also anyone with them, which is potentially significant. You could observe their body size, gait, mannerisms.
That said, there could be hundreds or even thousands of those backpacks sold in a given timeframe and geographic location. So someone from law enforcement has to watch every checkout video to see if someone they are interested in appears in it buying a backpack. If a card is used that gives a name right away, which can then be run down and checked to see if it leads anywhere.
Example: a group is seen. They buy a backpack. Pay with card. Law enforcement runs the name on the card. She's a kindergarten teacher, on video in the store with her husband and son, kid is in middle school. Unlikely to be either the perps or associated with them (and therefore not buying it for them). And you repeat again and again and again and again and again.
No BREAKING NEWS! No updates. No "activity." Just "boring" investigative work.
If cash is used then it's not a dead end. You have the video, first of all. You could always go talk to the cashier who was on duty if you find a potential video of interest. Does that guy come in often? Sometimes cashiers see the same regulars even if they do not interact with them. Maybe it helps, maybe not. Lots of things are crapshoots.
And if someone you already have your eye on as a suspect based on some other tip or evidentiary reason ALSO is seen buying a backpack AND with cash. It proves nothing. You cannot arrest or even likely raid them yet. But now you have a few "ands" that cause you to dig deeper. Literally, you have nothing actionable. You have no suspect in the official sense. But you think you are possibly onto something.
In this scenario the media claims "Investigators say they have no suspect." The case is going COLD!
No. It's just that these things take a lot of time and care, and resources are finite.
Yeah but what if someone bought the backpack for the perp?
Unlikely, and also not a dead end if it happened IMO. Here's why: that would mean someone else was involved to the point where they would go into Walmart and buy the backpack just so the future kidnapper wasn't seen on camera doing so. That is a LOT of care being taken.
Only someone fairly close to the perp would be trusted to do this, making them still potentially useful. If you see someone you have determined as being associated with your person of interest buying the backpack that is also useful. Plus, in the above scenario now you have a "shadow buyer" for a backpack who could talk for a million dollars. Especially if their only "crime" is buying a backpack for a friend/family member for whatever reason they might not even know for sure. Just unlikely, IMO.
One thing I think is certain: IF the kidnapper walked into a Walmart and bought that backpack, he was 100% on camera doing so. The trouble is knowing which purchase is "the" purchase. And casting a wide enough net. What if he went 5 states away to buy it? Or bought it a year ago AND several states away?
IMO we're in this phase, where investigators are working through potential evidence, which takes a long time.