The general consensus ,contrary to what Cobb themselves state on this issue, is that yes,an engineer can tell if the ECU has been reset.Now the ECU can be reset by just disconnecting the battery,but if something were to go wrong with the engine leading the ECU to be inspected by a Subaru engineer they could then assume that you flashed it(the ECU) and deny a claim.
It is their discretion.It is a risk you are taking.