I think they're being auctioned at black book, wholesale. I just bought my first VW (let alone TDI) two weeks ago. A 2014 with 29k miles on it, I assume it sat on a lot just like that, given the low mileage.
I got mine off Carvana. The Carfax on it leads me to believe that's exactly what happened. First owner had it for 4k miles, next had it for the rest. It went unregistered in 2015 and didn't get sold again until this January, as a VW certified pre-owned.
At any rate, it looks to me that many of them are in fact hitting dealers again. If anyone is considering one, check the tires, rubber hoses, and rust. If they all check out, then go for it.