Have you been looking for him during the night between midnight & 5am? That may be the only time he will make himself known. During the day, he may hear you calling, but be too afraid to answer you as their instinct is to hide and be quiet.
My parents had an indoor only cat who lived only 2 doors down that got outside and hid under their deck for 5 days. He didn't respond to his owners calls, nor could they find them (their deck was only had about a foot of clearance over the ground). My mother was in basement to watch some TV at 4 AM and saw him peak his head out to look into the basement window.
Cat maid to:
Puddles, female Himalayan (16 yrs)
Jasper, male Ragdoll ?? (approx 9 yrs)
Sweet Pea, female silver tabby DSH (approx 7 yrs)
Rose semi feral, a cpietra rescue, female tabby (approx 6 yrs)
Snowball RIP (1991-2005)
In a cat's eye, all things belong to cats.-English Proverb
“While we are free to choose our actions, we are not free to choose the consequences of our actions.” Stephen R. Covey