[2020] I think I accidentally attacked a biter in Peaceful mode - now what?

I play on peaceful mode since I find it more relaxing not having to worry about being attacked. I have almost 200 logged hours, but I have never played with attacks enabled.

My current playthrough is my furthest one to date. I've got a train network working and construction robots, I'm almost done researching advanced logistics. But two of my outposts are dangerously near biter bases. On my last trip to one, me and my train started getting attacked, even though I never attacked a single one of them. Now, does this mean that my train accidentally ran over one or could it also mean that my pollution has surpassed such a threshold that even "peaceful" biters can attack me?

In either case, what's the best solution to continue? If I kill off all the biters in that base, will other bases also be in offensive mode?