sudo zypper up
Loading repository data...
**Warning: **Repository 'Update Repository (Non-Oss)' appears to be outdated. Consider using a different mirror or server.
Reading installed packages...
Nothing to do.
Repos can become outdated (something wrong with the hosting site of the mirror that does not update anymore or similar). How to detect this? To give at least a warning about a possible not up-to-date Update repo, zypper checks the “date last changed” of the repo. When that is longer then a certain time (I guess in days) ago, it will print this message. The guess being that an Update repo probably has got an update within that time.
I assume that because 15.3 is just released, the Update repo being probably created some time ago and not having already any update (there beiing only a few non-OSS packages), you now stumble into this message.
It is really only a trial to avoid you using a stale repo for years without knowing.