Well, we are always thinking. Maybe just recalling wasn't enough of a change because people are always recalling things. Maybe there was always increased brain activity in one area, but it was a little less then the others so it eventually evened out in our perpetual thought process. Do we think when we sleep? Well, on a subconscious level, I suppose. So, if we monitored sleeping people, there is still a possibility we do a whole lot more subconscious thinking then conscious thinking, so the conscious thinking doesn't make the brain much more focused in the specific area of origin enough to draw attention.

Or maybe it's something else. Either way, memories are memories and they are stored in something, maybe even electric pulses or something. Does it really matter where they're stored? I admit, it could help relieve trauma and help discover lost minds to amnesia if we could tap into the depths of the human past, but hell. It doesn't matter too much, because trauma pretty much goes away with therapic techniques or time and amnesia'll probably have some form of cure or treatment in this age of technology.