Are there different options? Would the sensor only detect a single direction or all around it?
There have been security devices since way back that made a high-frequency echo pattern inside a building and if anything bigger than a cat moved, the change in pattern would trigger an alarm. But those only detect the fact of movement.
Look up PIR (passive IR) and IR reflect.
You could paint sections of wall with conductive paint and get capacitive sensing a ways out.
The simple way, IR beam interrupt using an IR led and IR detector in a tube.
What kind of floor does the room have? The right floor would know where feet are put.
Ultra-sonic, PIR, thermal imaging, radar, pressure plates, sound analysis, laser, a tin can with a wire, a human guard, a dog, ...
There must be many more.