At the point where the party is considering the glyph in the wall, and partly arranged along the steep staircase leading up from Falfysson's Cavern, they experience something distinctly eerie. It is a change in the air itself; abruptly, for those upon the stair (including the majority of the party, as I'll rule Lukas has just completed the hairpin), the air grows abruptly still.
This stillness is complete. Where before there was the sound of water from the cavern, the shuffling of dwarves' feet, an indistinct flow of air down the stairs - now there is nothing. Noise that was ignored before is now painfully evident in that it no longer exists.
Andrej knows at once that it is not a silence spell.
It is as if every ear in the party has suddenly closed up.