Sour of Hanoi: If you get the green- or blue-flashing notification (blue for spotpass specifically) while your 3DS is in sleep mode, then check to see if the colored dot icon is on your ACNL game icon. If it is, open the game and go right to your showcase. The only people standing in your courtyard/greeting area should be the most recent streetpassers or spotpassers. Check regularly so you don't end up getting too many new houses so that not all of the people show up in the courtyard. When you speak to the corresponding person, they'll tell you their showcase address.