This file is indexed.

/usr/share/logcheck/header.txt is in logcheck 1.3.17.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

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This email is sent by logcheck. If you no longer wish to receive
such mail, you can either deinstall the logcheck package or modify
its configuration file (/etc/logcheck/logcheck.conf).