Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

main_company.currency_id = env.ref('base.USD')
        test_sale_journal = journal_obj.create({'name': 'Sale Journal - Test',
                                                'code': 'TSJ',
                                                'type': 'sale',
                                                'company_id': main_company.id})
        env['product.pricelist'].search([]).write(dict(currency_id=main_company.currency_id.id))
        main_pos_config.journal_id = test_sale_journal
        main_pos_config.write({'journal_ids': [(0, 0, {'name': 'Cash Journal - Test',
                                                       'code': 'TSC',
                                                       'type': 'cash',
                                                       'company_id': main_company.id,
                                                       'journal_user': True})]})
        
        main_pos_config.open_session_cb()
        
        
        
        
        env['ir.module.module'].search([('name', '=', 'point_of_sale')], limit=1).state = 'installed'
        cr.release()
        self.phantom_js("/pos/web",
                        "odoo.__DEBUG__.services['web_tour.tour'].run('pos_basic_order')",
                        "odoo.__DEBUG__.services['web_tour.tour'].tours.pos_basic_order.ready",
                        login="admin")
        for order in env['pos.order'].search([]):
            self.assertEqual(order.state, 'paid', "Validated order has payment of " + str(order.amount_paid) + " and total of " + str(order.amount_total))