The dog in the article (a Vizsla) was refusing all food and treats (and had for four days) but was drinking normally and there was no vomiting or anything else. It said she was "active and healthy". The owner maintained that she couldn't have ingested anything unusual. The vets physical exam didn't reveal any problems. They then gave xrays which were normal and next a barium series and xrays over 8 hours. The dog still wasn't eating. Next they did blood tests, all normal. Finally they did an exploratory surgery which revealed a mass in her small intestine which when they opened it up they found a peach pit. The pit had absorbed a lot of moisture which made it similar in structure to the intestine walls, therefore it did not show up on the xrays. The pit had not caused a complete obstruction, allowing liquids to pass through normally. The pit was either causing her pain or making her nauseated. They said her behaviour was a typical response to a complete or incomplete obstruction.

So I guess the answer to your question is that they don't necessarily have to be lethargic.