about radar
and yes, they can 'see' through walls. hence the 'look down' comment.
since sensors do no evaluate data, the user has to use logic to determine what happens.
you see a car pass, and see, a red Toyota, with it's lights on, is passing from my left to my right.
an IR beam break see : break.
two sensors, one outside, one inside, may have one react first. first to react shows what side. the movement is.
however, if you knock on the door and stand there. and someone on the other side opens the door. how will you know ? and does that matter in this project ?
my first thought on the project was a PIR, so reliable that every alarm company uses more of them to detect people movement than any other sensor. door and window sensors don't detect people, they detect a state or condition of an object.
a capacitance sensor, like used in car alarms could be put on the door frame/wall and detect a hand moving to the door knob.
A person on one side can insert a wire and open the door on the other side.
lots of ways to add confusion and complexity to the basic premise. 