From researching, under VAG, audi is way more into this than the other brands. They won't just do this on a regular visit for an oil change, but if you have engine/turbo/trans problems, I have heard they go pretty deep to research the root cause. I have no idea how legally they could determine you used a piggyback to deny warranty, but I don't want to test it out lol. Multiple dealers who used to offer APR publicly then went to you gotta know a guy to now they just flat out dont do tunes, well flashes I should say. TD1 flag is a bitch and permanent.But I've heard that while they can't detect the device itself, they can determine by certain readings from the car that something had altered the normal range of outputs. I'm sure I'm wording that horribly. Basically, they can see that outputs fell outside the normal range that could have only resulted from ECU tampering.
On the VW side it seems (stories) guys can roll in with a stage 3 hybrid turbo GTI and get warranty work done so I have no clue how that gets OK-ed other than they are much more lax on the process.