Wild speculation here regarding detection of a piggy-back.
I truly believe VW can detect that a vehicle has *likely* been piggy-backed. If only for the simple fact that acceleration rates would be very strong, which I am sure these metrics are recorded in the ECU/PCM.
However, it wouldn't be slam-dunk for VW to defend against this kind of "speculative" or "circumstantial" evidence. It is very easy to deny TD1 because it is provable by VW that their ECU/PCM was modified.
If they wanted to say it seems to have been piggy-backed, people could claim they always have a tail-wind, were going slightly downhill every time WOT, or they had removed 400 pounds of crap from their car thereby causing it to accelerate faster.
When I had JB1 on my GTI, the power at mid-RPMs was far stronger than stock, there's no way it doesn't get recorded in some way that something is going on. Just tough to prove exactly what is up, I guess