Housebreaking adult toy breed
 

I have a female Papillon that has proven to be a challenge housebreaking. She had a rough start in life so here's a bit of history and hopefully someone has some advice for me.
She is an x-breeding puppy mill rescue (lived in cage from birth to 6yrs)
She lived with us as foster for 3 months to prepare her for adoption
She was on strict schedule for toileting and had no accidents
She was adopted although I cared for her on 3 occasions when the adopters were on holidays and she had no accidents
She was returned August 2013 as the family changed their mind
I decided to keep her and she settled back in with our family
On 4 occasions she has peed on the hardwood and the carpet since December. One time she had just gone out for a pee, then when I left the room she peed a tiny drop on the carpet. Another time a bigger puddle on them wood floor.
She has been to the vet and has no medical issues.
I'm confused. Have only owned cats or large breed dogs in past. Is this a small breed issue?:cry: