/etc/apt/apt.conf.d/90rkhunter is in rkhunter 1.4.0-3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | // Makes sure that rkhunter file properties database is updated after each remove or install only APT_AUTOGEN is enabled
DPkg::Post-Invoke { "if [ -x /usr/bin/rkhunter ] && grep -qiE '^APT_AUTOGEN=.?(true|yes)' /etc/default/rkhunter; then /usr/share/rkhunter/scripts/rkhupd.sh; fi"; };
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