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. 

_name = 'report.point_of_sale.report_saledetails'
    @api.model
    def get_sale_details(self, date_start=False, date_stop=False, configs=False):
        """ Serialise the orders of the day information
        params: date_start, date_stop string representing the datetime of order
        """
        if not configs:
            configs = self.env['pos.config'].search([])
        today = fields.Datetime.from_string(fields.Date.context_today(self))
        if date_start:
            date_start = fields.Datetime.from_string(date_start)
        else:
            
            date_start = today
        if date_stop:
            
            date_stop = fields.Datetime.from_string(date_stop)
        else:
            
            date_stop = today + timedelta(days=1, seconds=-1)
        
        date_stop = max(date_stop, date_start)
        date_start = fields.Datetime.to_string(date_start)
        date_stop = fields.Datetime.to_string(date_stop)
        orders = self.env['pos.order'].search([
            ('date_order', '>=', date_start),
            ('date_order', '<=', date_stop),