Unapproved visitors

Our Nanny has been working with our family, my 8mo for 4 months. She is not perfect but weathered the storm of our baby with serious separation anxiety and colic through some very difficult months. We are at a place where we trust her. Today I found out that she had her husband join her on an outing with my son. She did not tell me about this, someone who saw them out let me know. I am not ok with this. In fact, it makes me feel very uncomfortable. How should I approach it with her?

Edit to say thank you all for your perspectives and advice. I do plan to ask her about it and not come from an accusatory place. It’s pretty unlikely that they just ran into each other by chance but I’ll be open minded. I will be sure to set a boundary, at the end if the day I do not know her husband and I don’t want him spending time with my baby. That may work for some families but it won’t work for mine.