How do I find a "gap" in running counter with SQL?

I'd like to find the first "gap" in a counter column in an SQL table. For example, if there are values 1,2,4 and 5 I'd like to find out 3.

I can of course get the values in order and go through it manually, but I'd like to know if there would be a way to do it in SQL.

In addition, it should be quite standard SQL, working with different DBMSes.

In MySQL and PostgreSQL:

SELECT  id + 1
FROM    mytable mo
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    mytable mi 
        WHERE   mi.id = mo.id + 1
        )
ORDER BY
        id
LIMIT 1

In SQL Server:

SELECT  TOP 1
        id + 1
FROM    mytable mo
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    mytable mi 
        WHERE   mi.id = mo.id + 1
        )
ORDER BY
        id

In Oracle:

SELECT  *
FROM    (
        SELECT  id + 1 AS gap
        FROM    mytable mo
        WHERE   NOT EXISTS
                (
                SELECT  NULL
                FROM    mytable mi 
                WHERE   mi.id = mo.id + 1
                )
        ORDER BY
                id
        )
WHERE   rownum = 1

ANSI (works everywhere, least efficient):

SELECT  MIN(id) + 1
FROM    mytable mo
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    mytable mi 
        WHERE   mi.id = mo.id + 1
        )

Systems supporting sliding window functions:

SELECT  -- TOP 1
        -- Uncomment above for SQL Server 2012+
        previd + 1
FROM    (
        SELECT  id,
                LAG(id) OVER (ORDER BY id) previd
        FROM    mytable
        ) q
WHERE   previd <> id - 1
ORDER BY
        id
-- LIMIT 1
-- Uncomment above for PostgreSQL