functions.php

<?php

/**
 * Parent file
 *
 * @see https://github.com/WordPress/WordPress/blob/3.9.1/wp-admin/menu-header.php#L23
 * @param string $parent_file
 * @return string
 */
function orbis_timesheets_parent_file( $parent_file ) {
	$screen = get_current_screen();

	if ( 'orbis_timesheets_activity' == $screen->taxonomy ) {
		// Make sure the Orbis Timesheets menu is active
		$parent_file = 'orbis_timesheets';
	}

	return $parent_file;
}

add_filter( 'parent_file', 'orbis_timesheets_parent_file' );