While mulling over this case, I figured out the following:
Stacey either was carried from her residence, or she left under her own power. However, LE cleared her boyfriend via polygraph, so he didn't whisk her away. Also, anyone carrying Stacey would have had their arms full, literally. She weighed 180 pounds. Additionally, it's easy to attract undue attention from the neighbors in a quiet park. So it seems unlikely she was carried off without being noticed.
If she left on her own, the leg injury would limit her walking range. It's dubious she went into the lake; it's been searched with sonar. The articles above make it pretty clear the vicinity of the park was thoroughly searched.
Stacey was said to not have a license; however, no one said she didn't know how to drive. However, the neighborhood cars were accounted for, so she didn't drive off. This leaves the possibility that she hitchhiked. To hitch out of there, she would have had to stand in front of a solid block of modular homes. It's unlikely there's much traffic passing, this far in the boondocks. It's dubious she hitchhiked without being seen.
It seems I have proven Stacey could not have left the park unnoticed. The questions are, Who saw her leave? and Are they going to speak up?