I was running through Alftand Glacial Ruins and a stormcloak courier was in there.
I have been through here with most of my characters and never seen him in there before.

Has anyone else had a random encounter were you it shouldnt be.

I met such a courier down in Blackreach.

I saw one in Blackreach, as well. Anyway, they always spawn mostly near where you are, despite the location has sense for him to be there or not. It's just AI trigger.

I got one in Rannveig's Fast, poor guy fell for the trap. He later got out through one of the doors.