That's my take. I had a discussion with a tuner a few years ago regarding the piggyback tune on my Mini Cooper S. The piggyback tune works in conjunction with the factory tune, and if the piggyback tuner is removed, there is zero evidence of it having been there. But if you have a tune that replaces the factory tune, and then remove it before going to the dealer, there will be trace evidence of it having been loaded. But only if the service department specifically checks for the traces (I forget what he said they can do to find evidence). So I think in the majority of instances the dealership usually turns a blind eye until they're faced with an engine claim issue. Most who go with a custom tune just live with the fact that they might have to open their wallet if an engine issue develops that involves the dealership.
