System logging must be enabled.

The server error logs are invaluable because they can also be used to identify potential problems and enable proactive remediation. Log data can reveal anomalous behavior such as "not found" or "unauthorized" errors that may be an evidence of attack attempts. Failure to enable error logging can significantly reduce the ability of Web Administrators to detect or remediate problems.
The mod_log_config module provides for flexible logging of client requests. Logs are written in a customizable format, and may be written directly to a file, or to an external program. Conditional logging is provided so that individual requests may be included or excluded from the logs based on characteristics of the request.
Three directives are provided by this module: TransferLog to create a log file, LogFormat to set a custom format, and CustomLog to define a log file and format in one step. The TransferLog and CustomLogdirectives can be used multiple times in each server to cause each request to be logged to multiple files.
The server error log, whose name and location is set by the ErrorLog directive, is the most important log file. This is the place where Apache httpd will send diagnostic information and record any errors that it encounters in processing requests. It is the first place to look when a problem occurs with starting the server or with the operation of the server, since it will often contain details of what went wrong and how to fix it.