He pushed, pressed, groped about, but the glass apparently obeyed no one but Erik ...

Perhaps actions were not enough with a glass of the kind?
Perhaps he was expected to utter certain words?
When he was a little boy, he had heard that there were things that obeyed the spoken word! Suddenly, Raoul remembered something about a gate opening into the Rue Scribe, an underground passage running straight to the Rue Scribe from the lake ...

Yes, Christine had told him about that...

And, when he found that the key was no longer in the box, he nevertheless ran to the Rue Scribe.

Outside, in the street, he passed his trembling hands over the huge stones, felt for outlets ...