That's a brilliant question (if disturbing thought -- I mean, disturbing potential action by perp) Wondergirl -- to my knowledge, I don't think any one has asked that before. IF Sonia was abducted (and kept), it could better explain both why she was removed from her home and why the FBI was called in. But could someone really have managed to elude detection not just in relation to THREE crime scenes but, if I understand your questions correctly, a FOURTH? That is, another location where she may have been kept? It's a very shivering thought.....
But certainly I think the question is an important one and I really wish we could know the answer. (Although it's also awful to think about.)
The one thing I want to say about some of the conversation earlier above: there has been a fair bit of discussion on another site (Canadian) on Sonia's case where some posters have discredited the suggestion that the car doors and trunk of Sonia's car were open when the vehicle was first reported. I won't say more because I don't want to get into "hearsay" or to violate the TOC. But I will just say that I myself have been convinced by that discussion that Sonia's car was not found the way less careful media sites report or as the photos (post-LE involvement) suggest.
moo