And it was easier than I thought. It's the 1910 Salon de l'automobile, of course.
In this picture, taken from higher ground but just about the same angle, we can spot the same sign (a kind of winged O) which can be seen above the mystery car windshield". We can also spot the word "ASCENSEUR" behind the circular ROCHET SCNHEIDER sign which appears on both pictures.
So obviously the car stands under the "PIPE" sign (we can read "PI", backwards in the mystery car photo.
Pipre was a belgian carmaker from the early 20th century.
So I guess that this car is a Pipe. Maybe it's its slanted windshield that can be seen in the lower right of the picture, under the Pipe sign.