Source

File: wp-admin/includes/class-wp-filesystem-ftpext.php

public function exists( $file ) {
		$list = ftp_nlist( $this->link, $file );

		if ( empty( $list ) && $this->is_dir( $file ) ) {
			return true; // File is an empty directory.
		}

		return ! empty( $list ); // Empty list = no file, so invert.
	}