ok maybe not totally stealth, anything can be found IF somebody has the time to do so. If nothing is glareingly obvious right away that the car is re-mapped then i cant see them (VW/dealers) starting to question it. Like ive said before, when i had my ibiza i didnt have a SPS box. It went into 2 dealers on many occasions for warrenty work and they didnt mention anything. I spoke to the cheif mech there who wasnt really bothered about tuning and i told him it was remapped (after all work had been done!!) and he didnt have a clue that it was revo'd! Heres 1 more class example: I wrote my ibiza off back in late 2003, bad crash, car was modified quite heavily (wheels, suspension, brakes, exhaust, revo) nothing was said at all about the remap, weather or not he was looking for this, i dunno.
Basically tho yes if somebody can be bothered enough and has the tools and time and patience to look for the remaped software then yeah they might find it, depends how bumd they are gonna be about it.