Unknown Location

I see this with a lot of peeps, what does it mean?

What do you mean?
Like their current activity on their profiles?
It could mean several things like if the user belongs to a group you don't they could be looking at the section.
It can also be for changing passwords, uesrnames, emails, etc but I'm not sure if that's so.

A lot of the time it thinks you're looking at nudges.

I though its just when people idle after a while.