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. 

from odoo.api import Environment
import odoo.tests
class TestUi(odoo.tests.HttpCase):
    def test_01_pos_basic_order(self):
        cr = self.registry.cursor()
        assert cr == self.registry.test_cr
        env = Environment(cr, self.uid, {})
        journal_obj = env['account.journal']
        account_obj = env['account.account']
        main_company = env.ref('base.main_company')
        main_pos_config = env.ref('point_of_sale.pos_config_main')
        account_receivable = account_obj.create({'code': 'X1012',
                                                 'name': 'Account Receivable - Test',
                                                 'user_type_id': env.ref('account.data_account_type_receivable').id,
                                                 'reconcile': True})
        field = self.env['ir.model.fields'].search([('name', '=', 'property_account_receivable_id'),
                                                    ('model', '=', 'res.partner'),
                                                    ('relation', '=', 'account.account')], limit=1)
        env['ir.property'].create({'name': 'property_account_receivable_id',
                                   'company_id': main_company.id,
                                   'fields_id': field.id,
                                   'value': 'account.account,' + str(account_receivable.id)})