Get most recent row for given ID

Use the aggregate MAX(signin) grouped by id. This will list the most recent signin for each id.

SELECT 
 id, 
 MAX(signin) AS most_recent_signin
FROM tbl
GROUP BY id

To get the whole single record, perform an INNER JOIN against a subquery which returns only the MAX(signin) per id.

SELECT 
  tbl.id,
  signin,
  signout
FROM tbl
  INNER JOIN (
    SELECT id, MAX(signin) AS maxsign FROM tbl GROUP BY id
  ) ms ON tbl.id = ms.id AND signin = maxsign
WHERE tbl.id=1

SELECT *
FROM   tbl
WHERE  id = 1
ORDER  BY signin DESC
LIMIT  1;

The obvious index would be on (id), or a multicolumn index on (id, signin DESC).

Conveniently for the case, MySQL sorts NULL values last in descending order. That's what you typically want if there can be NULL values: the row with the latest not-null signin.

To get NULL values first:

ORDER BY signin IS NOT NULL, signin DESC

You may want to append more expressions to ORDER BY to get a deterministic pick from (potentially) multiple rows with NULL.
The same applies without NULL if signin is not defined UNIQUE.

Related:

• mysql order by, null first, and DESC after

The SQL standard does not explicitly define a default sort order for NULL values. The behavior varies quite a bit across different RDBMS. See:

• https://docs.mendix.com/refguide/null-ordering-behavior

But there are the NULLS FIRST / NULLS LAST clauses defined in the SQL standard and supported by most major RDBMS, but not by MySQL. See:

• SQL how to make null values come last when sorting ascending
• Sort by column ASC, but NULL values first?