Dog sneaks around house and goes potty

We have a 4 month old Doodle and overall her potty training has gone very well. She is very good about going outside and even comes to us barking when she needs to go out. However, once in a while we find that she has gone to the bathroom upstairs or in the basement.

We keep a very close eye on her so these trips are very quick. We restrict her from the upstairs and basement but with the kids, and parents, coming and going doors and gates get left open.

What we dont understand is why these accidents are happening without any attempt to tell us she needs to go out? It's only happened a few time, it's possible that we have missed her sign but we feel like she is sneaking off when the opportunity presents itself.