No telling. My dog ate an orange rubber chew toy, and the pieces did not appear until six days later. I have no explanation for that, but the dog never got sick. Something like a black plastic pen top might have been chewed into small enough pieces that it would be hard to see it in the stool of a large dog. If your dog happens to get sick anytime in the next several days, I would treat it as an emergency, but otherwise, the object will either show up in the next few days, or it won't.
Unfortunately, plastic is not digested. Swallowed objects have to make it through the stomach and intestinal loops. Any vomiting, diarrhea, mucus or dark colored stool?
If it hasn't passed in 72 hours, and you have gone through any stool to check, I suggest you go to the Vet and get an x-ray done before there might be major problems.
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