Missing (an) Important Canon Event, and is used when describing fanfiction.
It means that the story may use the entire canon universe from the original show/movie/etc, but one crucial event has been left out for plot reasons. Y'know, like undoing the canon death of your main character, and all that.
2. to mice/mouse something: To leave out a crucial fact for one's own purposes/advantages
Example two: "Geepers, my mother-in-law just moused out the fact that her daughter and I ever got married - treated me like a total stranger."