declare @T table(Number int)
insert into @T values (1),(2),(3),(13),(15)
select top 1 Number + 1
from @T
where Number + 1 not in (select Number from @T)
order by Number
Обновление:
Версия с использованием char (3) с добавлением нуля.
declare @T table(ID char(3))
insert into @T values ('001'),('002'),('003'),('013'),('015')
select top 1 right(1001 + Id, 3)
from @T
where Id + 1 not in (select Id from @T)
order by Id