2013 Gen 3 won't start sometimes, no juice, battery not the issue

I posted before about this but can't find it so I'm starting a new one. This problem has been going on since the middle of December. At first I thought it was a problem with the battery being drained but that ends up not being the problem. When it all happens, there are no lights, nothing shows, it's like the battery is either totally drained or disconnected.

It's been at the dealer more than I've had it in the past 45 days. They can't figure out what's wrong. They've even brought in a tech person from Toyota (Corporate??) to try to figure out what's going on. They think it's a fuse but when they test it, nothing shows wrong. They said they have to test it when it's acting up but it doesn't act up when they have it.

All that said, here is what it's doing. When I've had it, it starts fine and then, all of a sudden, goes totally "dark". No lights, no codes, no nothing. I've had to jump it (in the front) and then take it back in to the dealer. They've had to jump it to get it into the shop but then it starts fine after that. They've checked wires, connections, the starter button, connections from the battery on, and nothing shows up. They are at a loss.

Any ideas???

One thing they told me that might be of importance is once they tried to jump start it at the battery, not the contacts under the hood, but it wouldn't start.

The car only has 42,000 miles on it.