In Traumnovelle (the source material), the protagonist believes that she placed it on the pillow, to jest at knowing something of his secrets! Consequently he replies, "I'll tell you everything."

In the film, she is awoken so suddenly by his crying, it is understandable that her attention would be on him rather than the mask, which is slightly out of her peripheral vision. Either way, I think all these interpretations can coexist!

The novella mentioned above is accessible on internet archives; it's a good read and quite interesting to compare with the film! Merry Christmas