"suspect did not sufficiently invoke the right to remain silent when he stubbornly refused to talk:"
http://www.slate.com/articles/news_and_politics/jurisprudenc...
In this case, the suspect had spoken to the police, and during this didn't respond to a certain question / responded to the question with silence. If I recall correctly, the court reasoned that by first speaking, he gave up his right to remain silent. Moreover, he responded by silence rather that actively asserting his right to remain silent.
Not to say I agree with the ruling, especially because he was never mirandized. However, it is not clear cut that simply remaining silent isn't enough to assert your right against self-incrimination.