windowmime

(noun) The act of broadly gesturing and/or mouthing words to someone on the opposite side of a pane of glass (e.g., a window, car window, etc.), even if the other party could hear you if you spoke.
(verb) To engage in an act of windowmime.

"Betsy let me know she needed her car keys after she went outside by the key-usey-hand-twisty windowmime."