I agree that this is a matter of ethics, not company policy. The ticket belongs to the company, and to 'appropriate' it for personal use is stealing.

However, if you want to go with the company policy idea . . . it would depend on what your company's relationship with the travel agency or airline is. As mentioned above, a 'nonrefundable' ticket may still have some future value to the company, so it might not be totally lost to them. Therefore, for that reason too, I would not use if for personal travel.