So wait, you talked to a guard who saw her riding around at 2am on a regular enough basis to say she definitively would have gone on St. Julien? Or a guard who saw her the night in question who actually saw her turn left on St. Julien?
From the little we know of the route she actually took, it seems like she was trying to avoid the main thoroughfares. And while Johnston has a "bike lane", unlike Congress, it doesn't seem to me that coming out onto Johnston that close to downtown/UL on a Friday night, near closing time for bars, was the type of route she was going for. Johnston almost always has traffic, especially in that area, so if she made it that far I would imagine someone would have seen her. I suppose it's possible that doesn't preclude the possibility of something happening to her on St. Julien.