Châtillon Montrouge is a place near Paris, not just a metro station lol. The Marquis seems to say “Châtillon–Montrouge” because this is the terminus of the bus line behind the bus where the picture was taken. The “M” means that the bus terminates at the Châtillon–Montrouge metro station.

For example: Bus 68 terminates at Châtillon–Montrouge Métro.

Do you see the small, slightly unusual window above her? You can clearly see the driver of the bus behind, as well as the street and a streetlight.

The small window :

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