Problem
Objects in worlds start to clutter the world. These are items such as bikes, collectibles, plants, food and drinks. They can't be deleted as they are outside of a lot and even spawn on top ...
Ideas
Other affected objects are
poop, puddles, spawner (e.g. stones, frogs), plants, fruites, flowers (from neighborhoods). In Grim's Quarry and Conifer Station I had a lot of soybeans and soy plants in the ZoneObjetData of my apartments.
Haven't seen a spawner being added to ZoneObjectData other then the snowglobes but I didn't test many other hoods.
Plants can be any kind.
It probably also happens with homework, instruments and other stuff if you do what's in this thread: https://answers.ea.com/t5/Bug-Reports/NEEDS-SAVES-Objects-stuck-when-placed-in-world-after-loading-a-r/m-p/14193914
Haven't had a chance to test this case.
You found some very valuable info. Currently in the process of smoothing up your guide and putting it in my blog so that's what I'm busy with :D