Are logs in the LDF file being removed after a checkpoint?
If your database is in simple recovery the log might be truncated after a checkpoint. 

This does not always occur though for reasons discussed in Factors That Can Delay Log Truncation (the most obvious being an open transaction)

To see the inactive portion of the log you can use the undocumented DBCC log command.

DBCC log(0,1) /*First parameter is database id with 0 as current DB*/


or trace flag 2537 in conjunction with sys.fn_dblog

DBCC TRACEON(2537)

SELECT *
FROM sys.fn_dblog(NULL,NULL)


Of course there is no guarantee that the thing you are looking for will be there. It may be overwritten at any moment.