Haystack

The First Doctor reflected that looking for a needle in a haystack would be simplicity itself compared to his task of locating the source of a distress call amongst the Zarbi's anthills. (PROSE: The Lair of Zarbi Supremo)

The Eleventh Doctor conceded that looking for a Weeping Angel among statues was like looking for a needle in a haystack if the needle looked like hay, "a hay-alike needle of death in a haystack of [...] statues." (TV: The Time of Angels)

The Guerners' radar technology was allegedly sensitive enough to find "a beetle in a haystack". (PROSE: The Sleeping Beast)