Source

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

public function exists( $file ) {
		$list = $this->ftp->nlist( $file );

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

		return ! empty( $list ); // Empty list = no file, so invert.
		// Return $this->ftp->is_exists($file); has issues with ABOR+426 responses on the ncFTPd server.
	}