A silent source is a page an AI engine retrieves and uses while composing an answer, but does not link or name in the visible response. It shapes what the model says without ever surfacing as a citation. You supplied part of the answer and left no trace.
It is distinct from a ghost citation, where the engine does link your page but omits your brand name. A silent source is a step further into the dark: no link at all, only influence during retrieval. A mid-2026 Peec AI analysis illustrated this with arXiv, which it reported was pulled in during retrieval for unrelated queries (even a shopping question) and referenced several times without appearing in the answer. Treat that as a single-vendor, directional observation.
Silent sources matter because they break output-only measurement. If you only watch the citations an answer displays, you miss pages doing real work behind the scenes, and you miss when an engine quietly reorders which domains it trusts. It is the retrieval-side cousin of the visible-but-unclicked dark library effect. The fix is to measure retrieved sources, not just named ones.