Binaural streaming by frequency-proximity was investigated without subjective listener-feedback by modifying the scale illusion of Deutsch [J. Acoust. Soc. Am. 57, 1156-1160 (1975)] into a detection-task. Nineteen listeners had to detect one deviation within a repeating melody stream, while simultaneously presented with a randomized distractor stream. Every second note in each stream was presented to the opposite ear, requiring binaural streaming to detect the deviant. Listeners performed well in this test but adding interaural delay or timbre-difference let the listeners group by lateralization instead. This confirms the grouping by frequency-proximity. The method could be used to investigate binaural streaming in hearing-impaired patients, where interaural percepts might differ.