Source

File: wp-includes/class-wp-post.php

public function __isset( $key ) {
		if ( 'ancestors' === $key ) {
			return true;
		}

		if ( 'page_template' === $key ) {
			return true;
		}

		if ( 'post_category' === $key ) {
			return true;
		}

		if ( 'tags_input' === $key ) {
			return true;
		}

		return metadata_exists( 'post', $this->ID, $key );
	}