and how do I react next time they don’t greet me?

I started working at this department 3 weeks ago. I went into the office I now work at, greeted 2 coworkers I’ve already worked with, they looked at me, said nothing, kept talking to themselves.

How am I supposed to interpret that?

To me this is disrespectful, maybe you disagree?

Then, as I was working, I saw both of them staring at me. What am I supposed to do when that happens? To me this signals hostility and passive aggressiveness.

I separate private life from personal one but even I know that the least you can do is to greet your coworkers, unless you want them to quit.