Will my OEM brand dealer be able to tell that I have had my ECU remapped

The answer is "it depends". Some OEM dealers in small towns may not have sophisticated software to detect this, but in the main the answer is Yes, they will be able to see you have had your vehicle remapped. If you are using an OEM dealer for servicing, if you don't inform them and they upgrade the ECU software to the latest OEM version, your car ECU remap will be overwritten.