It's something that you should expect especially if you wear your watch frequently... Even if you are very careful wearing it, when you polish (wipe) your watch, some dust particles will somehow "scratch" your watch, that is how you got hairline scratches.. Probably that's what happened with your polished steel case...
It's not something that I would worry about..
But as for ceramic, its "quite safe" from scratches.. (even though I've heard some rare cases where 292 case got some hairline scratches) but on my ex 292, it was totally fine... what you should worry is if it got knocked very hardly... There are some cases where ceramic case cracked or even broke....
Best,
Sam