[{"_id":"taskId1"},{"_id":"taskId2"},{"_id":"taskId3"},{"_id":"taskId4"}]

db.collection.aggregate([{$group:{_id:"null",ids:{"$addToSet":"$_id"}}},{$project:{nonMatchingTaskIds:{$setDifference:[["taskId1","taskId2","taskId7","taskId8"],"$ids"]}}}])