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_session.py
help="The physical point of sale you will use.",
        required=True,
        index=True)
    name = fields.Char(string='Session ID', required=True, readonly=True, default='/')
    user_id = fields.Many2one(
        'res.users', string='Responsible',
        required=True,
        index=True,
        readonly=True,
        states={'opening_control': [('readonly', False)]},
        default=lambda self: self.env.uid)
    currency_id = fields.Many2one('res.currency', related='config_id.currency_id', string="Currency")
    start_at = fields.Datetime(string='Opening Date', readonly=True)
    stop_at = fields.Datetime(string='Closing Date', readonly=True, copy=False)
    state = fields.Selection(
        POS_SESSION_STATE, string='Status',
        required=True, readonly=True,
        index=True, copy=False, default='opening_control')
    sequence_number = fields.Integer(string='Order Sequence Number', help='A sequence number that is incremented with each order', default=1)
    login_number = fields.Integer(string='Login Sequence Number', help='A sequence number that is incremented each time a user resumes the pos session', default=0)
    cash_control = fields.Boolean(compute='_compute_cash_all',    string='Has Cash Control')
    cash_journal_id = fields.Many2one('account.journal', compute='_compute_cash_all', string='Cash Journal', store=True)
    cash_register_id = fields.Many2one('account.bank.statement', compute='_compute_cash_all', string='Cash Register', store=True)
    cash_register_balance_end_real = fields.Monetary(
        related='cash_register_id.balance_end_real',
        string="Ending Balance",
        help="Total of closing cash control lines.",
        readonly=True)
    cash_register_balance_start = fields.Monetary(
        related='cash_register_id.balance_start',