If someone at work randomly says they're in a relationship when they're around you does that mean they're suspecting that you're expressing interest in them? Or not necessarily?

One time I was at a woman's desk at work to work on something for about 30 minutes. I was only chatting about work related things pretty much. Then, sort of out of nowhere she said "I have a boyfriend". Then my response was just smiling and say "oh nice" and not much else.

I just feel like I found that confusing since it seemed random to me. Also, this has never really happened to me before when I've been around any women. It seems like they don't make that statement randomly.