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. 

iface_big_scrollbars = fields.Boolean('Large Scrollbars', help='For imprecise industrial touchscreens')
    iface_print_auto = fields.Boolean(string='Automatic Receipt Printing', default=False,
        help='The receipt will automatically be printed at the end of each order')
    iface_print_skip_screen = fields.Boolean(string='Skip Receipt Screen', default=True,
        help='The receipt screen will be skipped if the receipt can be printed automatically.')
    iface_precompute_cash = fields.Boolean(string='Prefill Cash Payment',
        help='The payment input will behave similarily to bank payment input, and will be prefilled with the exact due amount')
    iface_tax_included = fields.Boolean(string='Include Taxes in Prices',
        help='The displayed prices will always include all taxes, even if the taxes have been setup differently')
    iface_start_categ_id = fields.Many2one('pos.category', string='Start Category',
        help='The point of sale will display this product category by default. If no category is specified, all available products will be shown')
    iface_display_categ_images = fields.Boolean(string='Display Category Pictures',
        help="The product categories will be displayed with pictures.")
    cash_control = fields.Boolean(string='Cash Control', help="Check the amount of the cashbox at opening and closing.")
    receipt_header = fields.Text(string='Receipt Header', help="A short text that will be inserted as a header in the printed receipt")
    receipt_footer = fields.Text(string='Receipt Footer', help="A short text that will be inserted as a footer in the printed receipt")
    proxy_ip = fields.Char(string='IP Address', size=45,
        help='The hostname or ip address of the hardware proxy, Will be autodetected if left empty')
    active = fields.Boolean(default=True)
    uuid = fields.Char(readonly=True, default=lambda self: str(uuid.uuid4()),
        help='A globally unique identifier for this pos configuration, used to prevent conflicts in client-generated data')
    sequence_id = fields.Many2one('ir.sequence', string='Order IDs Sequence', readonly=True,
        help="This sequence is automatically created by Odoo but you can change it "
        "to customize the reference numbers of your orders.", copy=False)
    session_ids = fields.One2many('pos.session', 'config_id', string='Sessions')
    current_session_id = fields.Many2one('pos.session', compute='_compute_current_session', string="Current Session")
    current_session_state = fields.Char(compute='_compute_current_session')
    last_session_closing_cash = fields.Float(compute='_compute_last_session')
    last_session_closing_date = fields.Date(compute='_compute_last_session')
    pos_session_username = fields.Char(compute='_compute_current_session_user')