It's definitely possible to make it undetectable. The only way to detect it would be to monitor the electricity usage if they really know what they are doing and even then they might keep it below a threshold to make it harder to detect. If you believe nothing is completely undetectable then you are unaware of covert channels?
Well if you don't know what your doing I guess. Nothing is completely undetectable on a computer.
It's definitely possible to make it undetectable. The only way to detect it would be to monitor the electricity usage if they really know what they are doing and even then they might keep it below a threshold to make it harder to detect. If you believe nothing is completely undetectable then you are unaware of covert channels?