They could poop inside the box but digging it out by mistake when trying to cover it.

If you have more than one cat, one might be stressed by the other(s), especially if new.

Cat might be too old or too small to climb into the box, or not feeling well.

Some cats are assholes and doing it on purpose to show you they are not happy (is the box clean?).

My cat poops on the floor when there's too much litter in the box 😹 🤦‍♂️

My family had a cat who sometimes did that. The poor little guy just missed the box sometimes. The box would be clean too and it wasn't too small for him. We think he may have not been very bright because he also never covered anything. He scratched at the side of the box or the wall. His brother always covered it for him. He was very cute though.

Ask your significant other if he/she pooped on the floor. If not, it was most likely the cat.

We've got 6 cats and multiple boxes. I use webcams!

Just to clarify, are you asking so you can start containing the cat at night so that they only miss the box in an easy to clean room? Because trying to correct behavior only works if it's immediately after they poop on the floor, if any time passes they won't make the correct association.

How many cats do you have?

How many litter boxes do you have?

Is the litter mat kept clean and washed regularly (and not only because it was pooped on)?

There should ideally be at least 1 litter box per cat, plus 1 more to reduce the chance of stress pooping.

As others have mentioned, cats will also sometimes avoid going in a box if they perceive problems with it, eg too much/too little litter, box hasn't been scooped recently enough, etc.