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. 

assert len(self.ids) == 1, "you can open only one session at a time"
        if not self.current_session_id:
            self.current_session_id = self.env['pos.session'].create({
                'user_id': self.env.uid,
                'config_id': self.id
            })
            if self.current_session_id.state == 'opened':
                return self.open_ui()
            return self._open_session(self.current_session_id.id)
        return self._open_session(self.current_session_id.id)
    @api.multi
    def open_existing_session_cb(self):
        assert len(self.ids) == 1, "you can open only one session at a time"
        return self._open_session(self.current_session_id.id)
    def _open_session(self, session_id):
        return {
            'name': _('Session'),
            'view_type': 'form',
            'view_mode': 'form,tree',
            'res_model': 'pos.session',
            'res_id': session_id,
            'view_id': False,
            'type': 'ir.actions.act_window',
        }