Can anyone maybie share there opinion on what might of have happened to my cats !?

Ok so here's what happened, well first of all my cat had kittens three of em and we had had her in this backroom in my house . And by the way Its a backroom where u can leave the back door open. Ok anyways she had kittens in this box we left in there and we had em for a couple days in there , and there were starting to open there eyes just a lil and one day my gf decided that she was gnnna leave the back door open and I thought it was a bad idea cuz anything can go in and mess with them cuz there still very small and cuz there's this possum that always go around there .and she didn't listen to me and did it anyways. So when she went to go close the door later that day the kittens were missing !! Then we came to the conclusion that the possum took them and were very sad.cuz we did see that same possum that night in theback yard... But as days past we see are cat MIMI who had the kittens, come out from under this shed a few times and. we dnt normally see her under there she's always up and about Having a good time so we kinda are thinking that she moved them under but not sure. Sorry for the long story just trying to get every detail in. Hopefully I cam get some opinions from ya. Thanks -Johnny