Over the years we've witnessed some mystery poops from various dogs, once in an upstairs guest bedroom. That from a dog who normally did not go upstairs at all. It doesn't necessarily signal a problem, or it hasn't for us. It is sad to wonder what prompted the action, though. I'm always ready to blame it on something I've done or didn't do, like let them out enough or I missed their signals.
So where this was, had you already passed that area going in and out? It wasn't someplace she might have gone yesterday or overnight, from less pooping opportunity than usual yesterday?