I've run that experiment now, spoiler: It cheated with its pre-training knowledge https://georggrab.net/content/opus46retrieval.html

A real test is synthesizing 100,000 sentences of this slect random ones and then inject the traits you want thr LLM to detect and describe, eg have a set of words or phrases that may represent spells and have them used so that they do something. Then have the LLM find these random spells in the random corpus.

It could still remember where each spell is mentioned. I think the only way to properly test this would be to run it against an unpublished manuscript.