On GitHub, you could search for the: system/helper/db_schema.php file and lookup for each: order_id database fields. This will also show you the table source name that each orders are associated into despite if a customer has been logged with the order or without.
However, take note that this methodology may exclude the aside extensions you may had installed on your store which may also contain other order_id database fields. Otherwise, you may need to pull in a schema query from PHPMyAdmin (direct) to pull all the order_id database fields which will include all database tables.
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Programmer / Opencart Tester