DFT of a cosine/sine wave?
If a pure sinusoid is exactly integer periodic in the DFT's aperture, then the Sinc function, resulting from the rectangular window, will pass exactly thru zero at all but two points of the DFT results, thus appear to be invisible except for the two deltas.  If you vary the DFT window size by fractions of a period of the sinusoid, then it will show up, as you expect, since the DFT will no longer sample the Sinc when it crosses zero.