It's hard to say there is no merit to the theory given her history. After the loss of SSV Normandy (SR-1) in 2183, Commander Shepard was presumed dead with much of the crew and mourned throughout the Alliance. Her reappearance more than a year later alive and well under mysterious circumstances is bound to fuel rumors of a subsequent miraculous survival. However, as yet, there are no confirmed reports that Commander Shepard has survived.

Garibaldi, Alfredo "The Lost Shepard", Alliance News Network, 2188