I know that he's almost certainly not in the house, but we had that happen 2ce over the years. Mind you, we've rescued a lot of cats, so the percentage is very small.
One, a new cat re-homed to us, an adult, stayed quiet for nearly 2 weeks in a storage room. He was stuck between things.
Eventually, he made noise, and we followed it. Dh crawled in, between furniture and boxes...which is when he learned that he had some level of claustrophobia when he, too, got stuck...got hold of the cat, then had to be pulled out by his feet, due to being stuck.
Amazingly, the cat was fine, though hungry and thirsty.
It makes me wonder if a smaller cat, a kitten, might not get into a small space, and not move or make noise. In a coat pocket, in a vent, etc. It seems unlikely, but it happens.
So, I agree with you, he got out, but there's always a chance...
Hopefully, he won't go far outside. Have you spoken to the neighbors? They probably don't know him, to know he's yours yet?
Do you have a printer? Could you make a little poster?
Does he have a chip? If so, let your vet know if it gets dark and you haven't found him yet.
Would he know the sound of his food being rattled? As he gets hungry, maybe you could rattle the food/dish outside?
Please keep us posted. (((Hug!)))
Love All Life, Thank You For Posting! :)