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Memorization or Lucky Guesses: Detecting Short Sequences from Copyrighted Dutch News in LLM Output

Demonstrating that large language models have memorized copyrighted material is more feasible for high-volume publishers than for smaller outlets whose content appears less frequently online. This study explores how even short, repeated sequences–rather than full articles–can serve as evidence of memorization. Focusing on Dutch news sources included in the mC4 dataset, we test whether GPT-4 and mT5 reproduce excerpts from thousands of articles, including standardized editorial boilerplate. By comparing results to a post-training baseline and modeling memorization as a survival process, we find that repeated, publication-specific phrases are significantly more likely to be completed verbatim. The approach provides a means to detect empirical evidence of memorization in cases where full reproduction is unlikely.