even though it's not connected to power it will still be the hottest part?Power input.
iFixIt.com has tear downs of Macs.
Those Macs in their heat - a sine of over-heating? - a short story by Mister GGJstudios
i know it's bad for you but rarely sits on my legsDo you have the power plugged in (while the notebook is powered on) prior to putting it on your lap? If so, there are a few transistors in that area. They get hot in normal operation.
And for the record, the macbook pro is not really a 'laptop'. Better to have a lap desk...(ya, I know this is not what you were asking)