Who is emptying my trash and how do get them to stop?

Not sure about the NAPs because I don’t have that pack, but you can get a trash plant by leaving any trash pile on the ground outside. So if you repair something broken and get a trash pile, you can drag that outside. Or you can tell your Sim to empty an inside trash can, but cancel the action before they reach the outside trash can and they’ll drop it on the ground. Or I think if your Sim is mad you might have the option to kick over a trash can. Or a toddler repeatedly playing in cabinets will often leave a trash pile. Or I think dog poop left too long may also cause a trash plant if you have cats & dogs.