Option 1:

SELECT DISTINCT id +1
FROM mytable
WHERE id + 1 NOT IN (SELECT DISTINCT id FROM mytable);

Option 2:
;WITH Missing (missnum, maxid)
AS
(
 SELECT 1 AS missnum, (select max(id) from @TT)
 UNION ALL
 SELECT missnum + 1, maxid FROM Missing
 WHERE missnum < maxid
)
SELECT missnum
FROM Missing
LEFT OUTER JOIN @TT tt on tt.id = Missing.missnum
WHERE tt.id is NULL
OPTION (MAXRECURSION 0); 


Option 3: