Retrieval is a measurement instrument, and nobody reports its coverage

This can be seen as a special case of the CWA (Closed World Assumption): some systems assume that what they see is the whole world, i.e. that they have complete knowledge.

Your case describes a situation in RAG (Retrieval Augmented Generation), which is the special case where that happens inside a component of a bigger system, so the needed extra knowledge is actually available elsewhere in the same system.