What a Document's Absence From a Series Can Mean
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When a document expected to exist within an otherwise complete series simply is not there, the honest response is curiosity rather than an automatic assumption about why, since a missing document can result from ordinary loss, a deliberate removal, or simply never having been created by anyone in the first place at all.
Checking whether the surrounding series is otherwise genuinely complete helps calibrate how significant a single absence actually is, since a single gap in an otherwise thorough, well kept series carries more weight than the same gap sitting inside a generally patchy and incomplete collection kept in poorer condition overall.
Naming the absence plainly in a finished piece, rather than writing around it in silence, gives the reader the same information the researcher had to work with, and lets them weigh the gap's importance for themselves, rather than trusting a writer's silent assumption about what the absence most probably means.