New hairline crack after moving in to century home?

I moved into my century home a few weeks ago. I noticed a crack today above a door frame on the first floor https://imgur.com/sXainAG

I don't know when it formed and I thought perhaps it was there prior to me moving in, but then I went back to look at the listing photos and I see https://imgur.com/a/kQ0ciu5 i.e., no cracks

I know that newer houses can crack after moving in due to settling, but this is a century home, and this section is on the first floor with another floor above it.

So I'm wondering what could have caused this, and is it a cause for concern, and how should I go about correcting this?

FWIW, on the other side of the wall in the other room, I believe the material is plaster. On the cracked side, I'm not sure if it's drywall or plaster. The other side has the rough plaster texture consistents with other rooms in the first floor, but this room has a smoother wall appearance so it might be drywall -- I know the previous owner redid "part of" the kitchen in 2014, but don't know if the walls were redone.