So... I am a parisian and I know really well my own city. Let's look at a few screen captures:

screen capture 1This is clearly on the left bank, we can see the Invalides' dome and the Eiffel Tower.
screen capture 2
Again, I have no doubt Alexis is on the left bank
screen capture 3
Here we see the famous towers of Saint-Sulpice church (remember the Da Vinci Code ?). Helps a lot triangulation. I bet she's between Saint-Germain-des-Près and Sèvres-Babylone.
screen capture 4
Woof, Castle's crew couldn't have given me more clues :-) I can see the entrance of a subway station, this looks like the beginning of Boulevard Raspail...
screen capture 5
Guys, I can tell you Alexis is detained on the top floor of 236 Boulevard Saint-Germain in Paris...
screen capture 6
And to be even more precise, in that building.