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. 

return self.env['stock.warehouse'].search([('company_id', '=', self.env.user.company_id.id)], limit=1).lot_stock_id
    def _get_default_nomenclature(self):
        return self.env['barcode.nomenclature'].search([], limit=1)
    def _get_group_pos_manager(self):
        return self.env.ref('point_of_sale.group_pos_manager')
    def _get_group_pos_user(self):
        return self.env.ref('point_of_sale.group_pos_user')
    name = fields.Char(string='Point of Sale Name', index=True, required=True, help="An internal identification of the point of sale")
    journal_ids = fields.Many2many(
        'account.journal', 'pos_config_journal_rel',
        'pos_config_id', 'journal_id', string='Available Payment Methods',
        domain="[('journal_user', '=', True ), ('type', 'in', ['bank', 'cash'])]",)
    picking_type_id = fields.Many2one('stock.picking.type', string='Picking Type')
    stock_location_id = fields.Many2one(
        'stock.location', string='Stock Location',
        domain=[('usage', '=', 'internal')], required=True, default=_get_default_location)
    journal_id = fields.Many2one(
        'account.journal', string='Sale Journal',
        domain=[('type', '=', 'sale')],
        help="Accounting journal used to post sales entries.",
        default=_default_sale_journal)
    currency_id = fields.Many2one('res.currency', compute='_compute_currency', string="Currency")
    iface_cashdrawer = fields.Boolean(string='Cashdrawer', help="Automatically open the cashdrawer")
    iface_payment_terminal = fields.Boolean(string='Payment Terminal', help="Enables Payment Terminal integration")
    iface_electronic_scale = fields.Boolean(string='Electronic Scale', help="Enables Electronic Scale integration")
    iface_vkeyboard = fields.Boolean(string='Virtual KeyBoard', help="Enables an integrated Virtual Keyboard")
    iface_print_via_proxy = fields.Boolean(string='Print via Proxy', help="Bypass browser printing and prints via the hardware proxy")
    iface_scan_via_proxy = fields.Boolean(string='Scan via Proxy', help="Enable barcode scanning with a remotely connected barcode scanner")
    iface_invoicing = fields.Boolean(string='Invoicing', help='Enables invoice generation from the Point of Sale', default=True)