Both of my cats will sit and lie in and under something. That seems to be the first requirement for a cat when hiding or when he or she wants to feel safe. Something has to be above them so predators can't get at them from above.
This possible stems from the fact that cats live in a land of giants - us. Everything is taller than them. They are for ever looking up. It is hard to imagine what that feels like but it must be a bit intimidating sometimes.
Having found a flower bed with something above him or her my cats like a little opening in front to watch. A wall behind is ideal too. That provides all round protection with the front being observed. My cats feel safe when hiding like that.
When a cat dies I think they find a similar spot. It makes sense because when a cat is dying they are especially vulnerable and so need protection.
|Pippa hiding under a table|
Indoors the same rules apply. My boy cat hid under my desk when he first arrived at my home. It was almost impossible for him to get under it but he made it.
Another option for feeling safe but not for hiding is to find a high perch. Cat trees are designed to be tall with platforms for that reason. Athletic cats will find some extraordinary places, high up, that you wouldn't expect to suitable. These places feel safe for a cat because they put her above us and they no longer feel vulnerable.
I used to feed a stray cat I called Pippa. She was a bit timid. She would go to one of two places; under the coffee table that protected her from above and on top of the microwave that placed her high up in a protected position.