https://www.instructables.com/id/A-very-simple-proximity-detector/ I just clicked the link lol.
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Your hand. Price-free/original equipment, range-about 3 feet/1 meter...
Care to put some limits on your question? A therimin will 'detect' someone by their capacitance and emit a range of tones depending on how close they are. There are several IR light switches on the market at about $30? and up. You could put a microphone on the ground and amplify the sound of footsteps.