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. 

addons/point_of_sale/models/pos_config.py
PosSession = self.env['pos.session']
        for pos_config in self:
            session = PosSession.search_read(
                [('config_id', '=', pos_config.id), ('state', '=', 'closed')],
                ['cash_register_balance_end_real', 'stop_at'],
                order="stop_at desc", limit=1)
            if session:
                pos_config.last_session_closing_cash = session[0]['cash_register_balance_end_real']
                pos_config.last_session_closing_date = session[0]['stop_at']
            else:
                pos_config.last_session_closing_cash = 0
                pos_config.last_session_closing_date = False
    @api.depends('session_ids')
    def _compute_current_session_user(self):
        for pos_config in self:
            pos_config.pos_session_username = pos_config.session_ids.filtered(lambda s: s.state == 'opened').user_id.name
    @api.constrains('company_id', 'stock_location_id')
    def _check_company_location(self):
        if self.stock_location_id.company_id and self.stock_location_id.company_id.id != self.company_id.id:
            raise UserError(_("The company of the stock location is different than the one of point of sale"))
    @api.constrains('company_id', 'journal_id')
    def _check_company_journal(self):