So with a typical ECU, Yes, I would agree, However our (and other Kia/Hyundai ECU's) Where built very... lazily in that regard. I have it right from the Kia techline When my motor was replaced, that the integrity checks for the ECU's are largely based on RAM Storage instead of your typical flash memory, meaning that if they don't have power for a certain amount of time, all the information stored in that RAM storage, is just.... Gone. That is why resetting information such as the fuel adaptive is as easy as disconnecting the battery. As far as I have seen, the only things that are really stored Long term on the ECU's are Engine maps,
VIN information, and other stuff that typically does not change on a car, ever.
mileage, fuel economy, Boost numbers, and information like that is all variable, So it is stored in the short term RAM storage.
Your cars mileage is tracked in one of the CANBUS modules, so when the ECU looks at it to check apon ignition, instead of throwing up a Discrepancy, it just more or less deletes what it had last remembered the mileage at and just overrides it with what the canbus is reporting.
Now, all of this is information I got directly from the tech who worked on my car, Who IS "Stinger Certified" and is very knowledgeable on cars in general, Not the typical idiots that most kia dealers hire. He has dealt with this kind of issues before and he said that what I have stated above is more or less how the ECU thinks.
So Realistically If a dealer thinks your ECU has been tampered with, Id say with a 95% certainty,
its because you scratched the paint on where the ecu sits, and not that they can actually tell.
NOW, Take everything I have said here with a BIG Grain of salt, as I am not a kia employee and have not disassembled an ECU to verify RAM chips, PCB traces, or anything that would verify these claims. I am just passing on what I have been told. Don't shoot the messenger.