The phrase "mysterious stranger" refers to a person who is unknown and secretive, making it difficult to understand or know much about them. Full definition
Her skills and courage are soon put to the test when three mysterious strangers threaten to turn their vacation into a living hell. (uphe.com)
It is in this climate that two mysterious strangers enter the scene. (scene-stealers.com)
It is not surprising that Fossett reports between 1682 and 1701, «several local historians in the Orkneys and northern Scotland commented on the appearance of mysterious strangers in skin boats along the coasts.» (drtimball.com)