This is a good one to ask, but tricky sometimes. I've heard answers along the lines of 'we keep positions open perpetually' for architects, systems analysts, data scientists, and the like. So I think to myself, that's great! It is fun to think I'll be a valued hire, and the friend who brought me in will eligible for a nice referral bonus. However, I am starting to think that if a company appropriately values experienced, talented leadership in the most challenging sectors of development, then these roles don't need to be 'always open' any more than entry level positions. My guess is that there's always a catch with companies that give this answer.
So what's a good answer? I'd prefer: 'we weren't hiring, but X thought we should meet you.'