I have read an article from CNN.com that says the D. C. police have a tape which shows Levy having a "chat" with store clerk at a 7-11, the day before her disappearance.
The fact that this tape is consficated and reviewed and may be significant could be this.
When Levy is at her apartment, she is making her usual rounds in her place, while also sending an email. Maybe a person that knows the clerk or even the clerk has her address or something showing it or something that belongs to her as well. They knock and not thinking for a brief moment of the consequences of her actions, opens the door without checking. Perhaps at gun or knifepoint, she is asked to leave with the intruder, without the intruder having to step foot in her apartment and she is accosted to a waiting vehicle.
Another interesting note is a remark made by the former Chauffer for Rep. Gary Condit (the Congressman accused of having an affair with Levy), Vince Flammini. He says "She might have been a little weaker than people think", which suggest she could be gullible and open to aggression without thinking first. This could support the argument that the person knew just enough to convince her things were ok, even if the person had just met her, such as the store clerk or an observer at the store. Another is the question of who she knew outside the "circle" of her known acquantances. That does happen, sometimes a neighbor or a cab driver, etc. Not much, but maybe just enough to convince her that the person would cause any harm.
Consider that nothing was out of the ordinary in the apartment, but things were arranged as though she were still there, suggesting, an interruption of her time. Long enough to have her come to the door, too trusting, and become victimized, perhaps even going to a secondary crime scene.
Many programs warn us to always keep the assailant or attacker out in the open in plain view of everyone or around as many people in a given area as possible, whether daylight or dark. This increases the chances that if the assailant threatens to harm you, they will have to do it there, NOT at a secondary crime scene, which is most likely certain to allow the attacker to get away without positive identification.