I 'hear what you say' but I would not be prepared to take the risk myself.There is a lot of confusion regarding this between a piggyback device and a flash map. Where this device differes from a flash map, ECU data is not overwritten. Logged parameters are also not shown higher than stock, if the ECU did see paramaters higher than stock, it would either throw an error or simply trim back values to get back to stock power output.
As a result, you can pull a log from the ECU and everything looks within normal bounds.
It is very easy to make sweeping claims that your product will not leave a trace but I do not believe that Bluespark have enough knowledge of every manufacturers ECU to make that claim, furthermore there are many reports of similar devices (not from Bluespark I'm sure) on occasion putting cars into Limp mode precisely because they have exceeded a parameters limits. It is one thing to fool the cars systems to say boost the fuel rail pressure but it is quite another to manipulate the actual reading that the fuel rail reports back to the ECU, are you quite certain that your systems does this? Given that the ECU also records all manor or other data that is a consequence of the power boost are you saying that Bluesparks tuning box also manipulates all of these as well.
This is why although you can quickly remove the physical evidence of a tuning box I think a manufacturer can see the evidence if they look hard enough.