This plugin looks for patterns in files. It is capable of detecting
logfile rotation. If you tell it how the rotated archives look like, it
will also examine these files. Traditional logfile plugins were not aware
of the gap which could occur, so under some circumstances they ignored
what had happened between their checks. In a production environment 
this is not tolerable, that's why check_logfiles came into being.

WWW: http://labs.consol.de/lang/en/nagios/check_logfiles/