I finished Innocence Lost and reported back to Aventus. (I had previously completed Delayed Burial.)
I didn't wait or rest immediately - I just went about my business. It's been over a year game time, with plenty of rests and sleeps in plenty of places for plenty of time. I have never encountered a courier with a Mysterious Note nor have I been kidnapped by Astrid.
Any advice on how to initiate the "With Friends Like These..." quest and start the Dark Brotherhood quest line?

Thanks.

P.S.: I'm on PC