Sirhc's friend and STooge are correct, I spoke to two different BMW service technicians(coincidence they both worked for BMW) and yes they can tell if you've flashed your ecu. He explained you always flash forward you never flash back. In other words if you flashed forward to go to a specific performance tune, you have to flash forward again to revert to stock settings. The ecu shows it has been flashed twice although you're back on the stock mapping. Furthermore, he explained that they can't tell what you did on the first flash, but only that it has been flashed, so that leaves the door wide open for assumptions if something fails. He also explained that the software at the dealership won't be able to see this on their computer unless they run into a problem which requires manufacturer software that can see this. He didn't explain what problems would require this software, only that it exists. Hope this helps out with confusion about going back to stock mapping to try and fool the dealership. One reason you should go with Mtune if you're worried about voiding your warranty. At least their maps don't void warranty should they find out you've flashed your ecu. Also, mountune adjusts maps from data logging over a period of time to best suit your build setup. They also do this if you purchase a cobb v3 accessport from them, but I don't know if Ford will honor your warranty if it's not an Mtune accessport, but if I had to guess that's probably why the cobb accessport is $400 less than the Mtune accessport.