If I search for a string in a specific deck, it will only search in that one deck.
However, if I previously moved a card from that deck to another deck, that card will be included in the search. It may be important that the card was suspended before being moved.
Has this happened only once, or is it reproducible?
Can you give us a more concrete example of how to reproduce this? (along the lines of – card in full-deck-path-name – suspended – card moved to another-full-deck-path-name – searched for exact-search-string)
Does it still happen if you run Tools > Check Database before the search? Or if you restart Anki before the search?
When you reply, also include your full version info from – Help > About > Copy debug info