If an agent replies as a Public reply, the message written will be directly delivered to the client without passing through Unbabel, thus without being translated. Likewise, if #unbabel is removed, the message will also not be translated, it will simply be posted as an internal note.
Forgetting to add/removing #unbabel is a common reason for a translation request to fail. Since the result will be posting a regular internal note, there is no way to detect comments where this was a problem, so you should be extra careful and check the internal note for the trigger word if you suspect something is wrong.
Similarly, if you wish to post an internal note not meant to be translated, make sure #unbabel is removed, otherwise the content will find its way to the customer.