Sorry or your loss. I lost all my mallards in one night to a mink last October.
They were INDOORS, in a garage and in PENS and they still got killed. My chickens did not. Probably because of the difference in sleeping habits.
I, personally, would keep looking every day. It could be something simple...or could be a predator carried her off.
Does your other duck seemed "concerned"? Then there is a potential "problem"...keep looking! God Bless!