This file is indexed.

/etc/kernel/postinst.d/zz-update-grub is in grub-efi-amd64 2.02~beta2-36ubuntu3.

This file is owned by root:root, with mode 0o755.

The actual contents of the file can be viewed below.

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#! /bin/sh
set -e

which update-grub >/dev/null 2>&1 || exit 0

if type systemd-detect-virt >/dev/null 2>&1 &&
   systemd-detect-virt --quiet --container; then
	exit 0
fi

set -- $DEB_MAINT_PARAMS
mode="${1#\'}"
mode="${mode%\'}"
case $0:$mode in
    # Only run on postinst configure and postrm remove, to avoid wasting
    # time by calling update-grub multiple times on upgrade and removal.
    # Also run if we have no DEB_MAINT_PARAMS, in order to work with old
    # kernel packages.
    */postinst.d/*:|*/postinst.d/*:configure|*/postrm.d/*:|*/postrm.d/*:remove)
	if [ -e /boot/grub/grub.cfg ]; then
		exec update-grub
	fi
	;;
esac

exit 0