Gotta be in your house somewhere unless he grew 4 feet and learned to open a door.....
That's why I never ever ever leave my chameleon unattended. Way too many stories I've read where people step outside for a quick cigarette, or they go check on dinner for a few minutes, or they leave their chameleon outside in a bush for a minute while they run inside to use the restroom.... All to come back to a lost chameleon. Try turning off all the lights in the evening, and leaving only one light on - his heat bulb. Also put some insects in a tub in a room or two, and make sure the tub is see-through. If he is hungry, the insect container will attract him. If he is cold, the heat lamp will attract him. I'd be worried about him getting water though....
Best of luck on your hunt