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. 

group_by = fields.Boolean(string='Group Journal Items', default=True,
        help="Check this if you want to group the Journal Items by Product while closing a Session")
    pricelist_id = fields.Many2one('product.pricelist', string='Pricelist', required=True, default=_default_pricelist)
    company_id = fields.Many2one('res.company', string='Company', required=True, default=lambda self: self.env.user.company_id)
    barcode_nomenclature_id = fields.Many2one('barcode.nomenclature', string='Barcodes', required=True, default=_get_default_nomenclature,
        help='Defines what kind of barcodes are available and how they are assigned to products, customers and cashiers')
    group_pos_manager_id = fields.Many2one('res.groups', string='Point of Sale Manager Group', default=_get_group_pos_manager,
        help='This field is there to pass the id of the pos manager group to the point of sale client')
    group_pos_user_id = fields.Many2one('res.groups', string='Point of Sale User Group', default=_get_group_pos_user,
        help='This field is there to pass the id of the pos user group to the point of sale client')
    tip_product_id = fields.Many2one('product.product', string='Tip Product',
        help="The product used to encode the customer tip. Leave empty if you do not accept tips.")
    fiscal_position_ids = fields.Many2many('account.fiscal.position', string='Fiscal Positions')
    default_fiscal_position_id = fields.Many2one('account.fiscal.position', string='Default Fiscal Position')
    default_cashbox_lines_ids = fields.One2many('account.cashbox.line', 'default_pos_id', string='Default Balance')
    @api.depends('journal_id.currency_id', 'journal_id.company_id.currency_id')
    def _compute_currency(self):
        for pos_config in self:
            if pos_config.journal_id:
                pos_config.currency_id = pos_config.journal_id.currency_id.id or pos_config.journal_id.company_id.currency_id.id
            else:
                pos_config.currency_id = self.env.user.company_id.currency_id.id
    @api.depends('session_ids')
    def _compute_current_session(self):
        for pos_config in self:
            session = pos_config.session_ids.filtered(lambda r: r.user_id.id == self.env.uid and not r.state == 'closed')
            pos_config.current_session_id = session
            pos_config.current_session_state = session.state
    @api.depends('session_ids')
    def _compute_last_session(self):