Stuff happens...

[Ben]

...that you're not aware of.

What was happening? Also, what's the thinking behind mentioning it: is it a reminder that if people want to make listen/spot etc checks they can?

[Alex Newall]

I will answer the question but first, a warning:
In general I will avoid discussing certain aspects of GMing in depth, especially any questions about what I would have done if x hadn't happened etc. This is because from experience it leads people to start deconstructing it as they listen rather than engaging with the story and they would have been happier not knowing.

So, with that in mind I won't be telling you what was happening but I will say why I bothered mentioning it. A few reasons:
1) I don't like the idea that just because a character is off the battlemap they don't matter, so often I will keep them in combat time and doing stuff behind the scenes.
2) Mentioning that stuff to the party helps remind them that there is world outside of the area they are fighting in. It sounds daft but little touches like that help people avoid getting trapped thinking only about the next turn. I find fighters have a hard time with this problem because the way their build works encourages them to think that way.
3) Sometimes things happening behind the scenes are time dependent and I need to make sure I stayed in turn order, otherwise when GMing I can lose track of how far along in a task an npc is.
4) I have a bad habit of saying everything that enters me mind the second it does.

Hope some of that helps. Sorry I can't give any more info.