[{_id:1,animal:"cat"},{_id:2,animal:"mat"}]

db.collection.aggregate([{$facet:{data:[{$group:{_id:0,animals:{$addToSet:"$animal"}}}]}},{$project:{data:{$cond:[{$gt:[{$size:"$data"},0]},{$first:"$data"},{animals:[]}]}}},{$project:{data:"$data.animals"}},{$project:{_id:0,missing:{$setDifference:[["cat","mat","rat"],"$data"]}}}])