When Charles is playing music with Jan, she is shown as being 3 floors down and across the very large courtyard. After she stops playing, she knocks on his door 17 seconds after last being shown sitting in the window. During this time at some point she slipped away from the window (despite Charles seeming to be constantly looking at her), found a pen and paper, wrote her note, called and caught the elevator, and walked all the way around to Charles' side of the building to his door. Just the elevator ride would take this long, it is not possible for her to have done all of that in that short a time.