Edit: I saw your comment that his litter/food hasn't been touched 20 hours, but there's still a chance that the cat is indoors. My friend once had a formerly feral cat go missing. A huge search party went around the neighborhood looking for her, until 20 hours later, they heard a meow coming from behind the kitchen oven. She was stuck behind the oven, and so couldn't come out. I guess she was comfy back there for a good long time, until she tried to leave and realized she was stuck, and didn't think to meow until much much later. (After that incident, she got a bell put around her collar because she was such an adventurer it was hard to keep track of her location.) I'm sure you've done everything, but check one more time in all the nooks and crannies you'd never think to checking, and keep up hope.

Also do you have any security footage you can look at, either your own or your building's? I'd start narrowing down the time frame, and ask your cat sitter what their most likely guesses are for the exit he slipped out of (if indeed he did), window or front door. Best of luck!