Are you using a microprocessor to process the signal? Or are you thinking along the lines of a stand-alone sensor? One that provides a binary person/ no person output?

Perhaps you could use an ultrasonic sensor. This one has a range of 1”-13’.

A simple microprocessor can process the output and provide your binary output. I’d recommend either an Arduino Nano or an ATTiny85.

Or you might be able to design a stand-alone circuit.

No microprocessor. Simple voltage comparator - using LM358 for this. Simply turn on if voltage crosses some predefined threshold (person comes up at some distance).

 

Then using an ultrasonic sensor might be problematic. Their output is a pulse, whose width represents distance. I’d say use something to convert the pulse to a voltage, but I don’t think you can. There is no fixed period over which you measure.

You may be able to use the rising pulse to start charging a capacitor. Then the falling edge could trigger your circuit to read and compare the voltage on the cap. Unfortunately, I can only theoretically describe the circuit.

I’ve replied because an ultrasonic sensor will provide you the range you desire and perhaps is worth investigating.