Looks like it was dinged by something.
I have never had to use Subaru's warranty, so I don't know how customer friendly they are. They may cover it, or the may claim it's damage.
In either case, I'd get a match stick and sand the rust off. Put on a dab of touch up paint. Let it dry. Put on a dab of clear coat. Since it's under a seal, you may not even want to fool with the clear.
Just need to paint it to keep it from rusting further.