How do I pick up garbage that I can't see?

I also had something like that happen, although it was with a pizza box and it just kind of disappeared when the sim put it on a table. The sims were still able to use it but I couldn't see or click on it even when I knew where it was placed.

I think that if your free will slider is set on full ( so that the sim can do something on their own even if you aren't giving them tasks) they will eventually pick the invisible garbage and throw it out. If not, maybe going into build mode and clicking on the floor around that area could help you find the object?