The husband threw the Get to his wife and it fell between barrels. When they checked they found a Mezuzah instead. The assumption is that the husband is not available for clarification and the Gemarah conclude that a mouse took the Get ( how about the Mezuzah?) and therefore from the Leshon of the Gemarah it is meshma that she is divorced and probably allowed to remarry.The question we dealing here is a sofek D'Eraytha and we should go LeChumarah since we are not sure if the husband through a Mezuzah instead of the Get and really we don't have a raya that indeed it was a valid Get.
The Rambam and Shulchan Aruch (EH 135:5) write that she is indeed Safek Megureshes.