Phillippe, your entrance is much more beautiful than mine. Thank you for the invitation to see what it looks like on the inside - next time I'll know better than to try to visit in Fevrier! ;)
I read about the secret entrance in Frommer's Irreverent Paris, a wonderfully quirky guide on Paris that focuses on off-the-beaten path places in Paris while giving real opinions (not just standard guidebook remarks & recommendations).