I am having the same problem with Teeka! She potties outside great and tells me by whining at the door. Some times she does tell me when she has to go out to poo but if I don't catch her que, she does it in the house behind the recliner!
I feel some of it is my fault because she is whining and I am not paying attention.
It is kind of funny that Teeka is almost 11 months old and not completely trained and Tatiana is 5 months and completely trained......no accidents!
The difference is that Teeka is outside trained and Tatiana is pad & outside trained. When I am busy she just goes to her pad!
I don't know if this helps but it is an insight to why my Teeka does it. Good luck!