I think you're right. I would avoid using out or ahead. Let's take a look at each. There are warning signs out for the Korean economy. This doesn't make sense to me, nor would I ever say it this way (even if it is correct). There are warning signs ahead for the Korean economy. This is something I might hear a native English speaker say, but I still feel it doesn't make sense. Warning signs don't came "ahead". They come now and warn of something in the future. Therefore, I would choose to say it exactly as you suggested: There are warning signs for the Korean economy.