It could also be in the brain yes, we don't know. One could argue the bible would say "In your head" if they meant brain but we don't know yet.
Strong's G3359 - metōpon (forehead) is used literally or figuratively in an internal sense.
So forehead implies either a spiritual sense and/or the brain. And if you have been watching as you are commanded to do... you ought to know very well that implantable microchip is the next big thing.