Source?
Are the people doing the coding/mapping saying it can be removed and the dealer cannot see it was ever done? As that doesn't surprise me, they are the people who are making money from doing the coding.
Or has a BMW tech or ex tech confirmed that once removed it cannot be seen?
The master tech at my dealer showed me what happens, as soon as it's plugged in to their system, it flashes up and they have to acknowledge it. Even if it's been previously coded or mapped, it still flags and once flagged it remains on the BMW system.
Just because BMW can see it, doesn't mean they're going to tell you they can see it. But it will be logged, and relied upon in the event of any warranty claim.
If there's a coder out there who can give me a GTS flash, and guarantee that BMW cannot see it, and agrees to pay for a new engine/gearbox in the event that BMW do see it, let me know as I have money waiting. But I highly doubt such a coder exists, as they won't want to take that risk.