There's lots of places in my house for my cats to hide, so I don't panic too much if I don't see them for a while. Lately though, Callie doesn't even hide. 9 times out of 10 she's sleeping on one of the beds upstairs. She's gotten really lazy. lol. But I know when I don't see my cats for a while and I don't see them upstairs anywhere, they're probably in the crawl space in the basement. Callie also likes to sit on some kind of power box for something that my dad has downstairs. He keeps it on so it's always warm and she sits on it a lot in the winter. Puffy used to sit behind the TV sometimes. And she also liked to sit/sleep on the towels in the bottom of the bathroom closet. Callie is sometimes under one of the beds.
The only time we worry is if we've literally not seen one of them for more than a few hours.
One time I had to bring Puffy to the vet, and I think she knew something was up because I took out the carrier. Then of course when I needed to start getting her ready to go, I couldn't find her ANYWHERE. I looked in the bedrooms, in the basement, all over the living room and computer room, and I was calling her and I couldn't find her anywhere. Then when I sat down to put my shoes on, she came walking out from the living room. I have no idea where she'd been!