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. 

"""A Point of Sale is paid when the sum
        @return: True
        """
        for order in self:
            if order.lines and not order.amount_total:
                continue
            if (not order.lines) or (not order.statement_ids) or (abs(order.amount_total - order.amount_paid) > 0.00001):
                return False
        return True
    def create_picking(self):
        """Create a picking for each order and validate it."""
        Picking = self.env['stock.picking']
        Move = self.env['stock.move']
        StockWarehouse = self.env['stock.warehouse']
        for order in self:
            address = order.partner_id.address_get(['delivery']) or {}
            picking_type = order.picking_type_id
            picking_id = False
            location_id = order.location_id.id
            if order.partner_id:
                destination_id = order.partner_id.property_stock_customer.id
            else:
                if (not picking_type) or (not picking_type.default_location_dest_id):
                    customerloc, supplierloc = StockWarehouse._get_partner_locations()
                    destination_id = customerloc.id
                else:
                    destination_id = picking_type.default_location_dest_id.id
            if picking_type:
                pos_qty = all([x.qty >= 0 for x in order.lines])
                picking_id = Picking.create({