Any decision made with all parties knowledgeable and in agreement avoids the ethics. The minute " no one will ever know" creeps in it is probably unethical....not trying to point fingers at OP here...

In my company, we book non- refundable travel knowing that we run the risk of ening up with unusable tickets. If a manager wanted to "grant" such a ticket to an employee for personal use i would ask Finance to approve and document. This might be a taxable event btw.